Because she views them as those who don’t have the right to it. Like they’re not her kin anymore … 
It would be more accurately to state it’s a political stigma of xenophobia.
If she’s NOT xenophobic, then that means she’s even more worse off — as being simply described best as a mere racist & rather immensely discriminatory to those who don’t bend the knee to her doctrines. 
Until the Catalcysm when the Shen’dralar were grudgingly allowed in on a short leash any use of arcane magic was a death sentence. The highborne were forced to forgo their old privileges and cultural practices and to integrate into what would been the peasantry class of the old empire. That is how big of a stigma developed over the highborne and their magics—And eventually they rose up and were banished from kalimdor. In honesty, that was a death sentence as they did not even know rather or not there even was any land across the great sea still at that time.
The elves are all awful about one another, as stigma and resentment runs deep. Don’t forget that Liadran was all to happy to mean girl gossip about how night elves are tree dwelling savages the second she got a chance to in that recruitment scenario.
Makes one wonder how they plan to bring all the elves together.
Not all of her own people actually fought the Legion, and she exiled a ton of elves who did fight the Legion.
I think at this point being a prickly undiplomatic leader is just baked into her DNA as a character.
Was that ever in doubt? You don’t take over control over a massive empire in your 20s and refuse to relinquish power for millenia, and exile everyone who refuses to live in the lifestyle you and your husband find ideal if you’re a tolerant, liberal leader.
100%. She was built with the uncompromising amazon warrior woman archetype in WC3 and over time that did not do a lot of good for her as the game setting (and her own people’s depiction) evolved around her but she mostly stayed the same.
In some ways I kind of like it, as it shows the problems a 10,000 year old person should have with a world changing as fast as it has the past few decades.
I mean, the Nightborne were pretty gucci with the Nightelves until Tyrande had stuck her nose up at them … Not to mention, the Nightborne & Blood Elves seem to be on really good terms — Even so much as having Lor’themar & Thalyssra get married
It’s been displayed that many (not all) Blood Elves don’t have a problem with the High Elves as much as they do with them …
As for the Nightelves — Malfurion tends to be far more diplomatic than Tyrande, despite still having some degree of stigma as many of their people do.
However, as you stated: In Cataclysm they rewelcomed the Shen’dralar into their ranks, hence why I made a short nod towards the expansion in an earlier comment of mine →
Which is regarded to another comment I made shortly later on →
But yeah
Tyrande is a working progress but I find Malfurion to be the more welcoming one than her. However she has displayed a closer bond with those within the Alliance, so I suppose that’s a start. lol
The Horde’s elves appear to be far more friendly, welcoming & less disciminatory than the Alliance elves are … 
Some of this is because the night elves are victims of the redefining of what arcane magic even is. When it was a corrupting wannabe warhammer winds of magic force that drove its users into hubristic madness and attracted demons that wanted to possess you and cause havoc the old night elf position made a lot more sense than it does now with void, fel, and arcane split apart.
Eh, I’d say that’s an oversimplification of a copout excuse to use @ them being awful and/or discriminatory to others.
Or, obviously, tbcs power stature was retconned because shadowlands did nothing but tell us how unstoppable elune’s champions are.
Elune’s champion in another god’s realm in the shadowlands was more powerful than all of that god’s champions in his own heckin’ realm.
It’s hard when we already know that regardless of who ever Elune is - of all the races in the cosmos, the Night Elves are her favored race.
There is a level of separation there. A barrier. If I am any other race, I am lesser in her mind. Much less if I am Horde, where I kill her favored Race all over the place.
Even if we had a story about Elune with 0 Night Elves involved- if every race except Night Elves was involved - everyone there is less important to her than Night Elves.
Mind you - I acknowledge it would be a slap in the face to Night Elf fans for Elune to say “I don’t have favorites anymore” or something like that. Many Night Elf fans would surely lose their minds if Elune suddenly viewed them as equals with the other races.
I just want to know why everyone else has to be chained to elune, why can’t they have their own religions worked on instead of being absorbed into elune worship
You know what? I see your point, It is a fair point, once elune stated night elves are her favored race, other people worshipping her would be automatically “less” than that.
It is sort of a weird thing. Their legacy as descendants of an ancient empire kind of got pulled from them by the Nightborne. So, take Elune liking them most and night elves sort of loose all definition. Elune’s favor and the old quasi-atlantian empire brought low by hubris were the two defining points for the Kaldorei pretty much the entire history of their inclusion in the franchise.
Generally I am happy about any kind of new lore we get, as long as it makes sense. I definitely don’t mind seeing more of Elune.
I imagine it’s more of a ‘gradient of favoritism’. Like, even among the Kaldorei, I imagine there might be some slightly different levels of ‘opinion’ towards, when comparing the ‘Priestesses/Druids of Elune’ who obsessively worship her. Compared to, say, Night Elf Warlocks/Mohawks and Death Knighdorei. I doubt she views the universe as a binary Kaldorei vs literally everyone/everything else.
I mean, arguably she has an even stronger affinity, specifically favoring the “Night Warrior(s)” than the ‘overall general Kaldorei’.
I don’t think it’s quite apt to “If you ain’t first, you’re last” the situation.
I like Elune lore because it’s mystically vague, the name has a soothing auroral quality, and I enjoy the ingame iconography and ‘unoffical’ fan depictions, inspired by her typical entourage(like Priestesses of Elune). I just think it’s neat.
/[Kaldorei_BodyType2_Standing_Idle_Bounce_Animation] 
I wouldn’t mind some more Elune lore only if they focus on other groups that have some sort of connection to her that are not Night Elves.
We got the Tauren / Highmountain Tauren via Mu’sha for example. Or the group of Zandalari druids that are “balance” druids.
I still headcanon that the ‘tidemother’ in Kul Tiran culture is Elune. Given how connected the moon is to the tides in real life. Something that was referenced in Cata era Darkshore where you banish some water elementals by using the “Orb of Elune”. Plus it is in character for Azshara to impersonate Elune. But then I’m sure certain posters will cry that humans will take another thing away from X race.
I want more of this lore expanded. It would do a great deal to mend the rift for Horde folk who see Elune as some Alliance supporting force in the cosmos.
Night Elves are descended of Trolls. The Lunalai don’t belong to Elune’s favored, they belong to the race that spawned Elune’s favored race. The progenitors.
Blizzard could go the route of showing the Lunalai as “we aren’t Elune’s favorite because we know her better, and she doesn’t like that”
Sort of like the Trolls are Elune’s kids that have a strained relationship, but the kaldorei are Elune’s grandkids that she likes better.
I feel like those folk would just be mad that an alliance god got screentime when other horde gods could have
(This is Cursewords on his Zandalari Druid)
I am one of those folks, though. As a Troll Druid Player, I was stoked to see the Lunalai. A link to something more cosmic.
I don’t “RP” as some Lunalai, but the idea and concept of them being around is a great spring board, imo.
Especially since the Zandalari as a whole view the Lunalai with less consideration. The Zandalari consider almost anything a Loa, but not the deity of the Lunalai.
As a Zandalari Druid Player, I would like to see the Elune connection hammered out. Being the favorite isn’t always important if we have a better understanding.
If the Lunalai come from a Trollish tradition that followed Elune but would not go towards the path of becoming Elves, and thus, fell short of being the favorite - at least Trolls could say we declined being her favorite because we wanted to remain Trolls.
I personally like Elune and how the Night Elves are her “favored children”. I think that’s really cool and part of their racial fantasy, you are favored by a god. That’s badass.
My issue with Elune, or rather the way Blizzard has written around her is that in order to write this whole “Night elves are her favored children” they have crapped on other races and their religions somewhat.
I think it would be cool to see other races get attention to their religions. Like the Tauren with An’she, build up An’she to be a cool god on par with Elune and the Tauren are his favored children and what not.
Yeah, it’s mostly the combination of making the NE her favorite children and also making her a god of everyone.