The majority of people who play Horde do so because they like to raid, do M+, etc (not everyone, but it’s definitely the better faction for finding groups for high end PVE content).
So the reasons people have for playing Alliance definitely have to be more varied and interesting. So why do you play Alliance then?
For me, it’s because I like being able to wear shoes, and Alliance has more races that wear shoes.
One word…
DWARF
Humans.
But Orcs are admittedly the best race in WoW.
Grew up wearing Carebear pajamas?
Cute purple kitty.
Better racial traits.
I’m never the bad guy no matter what.
Wildhammer/Dark iron/Iron forge Dwarf
Night elves
Both versions of Gnomes.
Gnomes
The best race
I really don’t care for the horde races appearances.
my friends play alliance
No benefits to switch.
Fast BG queues, more honor, more active WPvP scene… at the cost of performance in epic BG’s.
No vulpera.
This toon is based off of a D&D character I made around 20 years ago. I don’t have 1 main, I also have a horde toon that I also work on & that’s also based on another D&D character I made, but Sernius just happens to be my favorite D&D character that isn’t a bard.
I like a couple of the alliance races. But that’s the main issue with me, a couple. Alliance needs some more appealing races
Great takes on classic RPG races that actually look good in transmog, especially night elves and dwarves. I’d throw worgen in there if they could spend more time in human form.
The amazing capital cities to explore, especially Ironforge, Darnassus and new Stormwind. Orgimmar, Thunderbluff and even Zuldazar look terrible aesthetically.
The questing zones. Westfall Defias plots, Duskwood’s haunted past, the nooks and corners of Ashenvale and Feralas.
Faction leaders that are actually good people doing good things.
Horde player’s being such scum back in vanilla.
That’s what got me started at least. What keeps me? I don’t raid on the edge, most horde races look terrible in gear, and it’s pretty clear from the story that either the Horde are evil or too cowardly to do the right thing consistently.
I was former Horde, switched to Alliance because I like the races, cities, designs, and vibe a whole lot better.
The short bg queue times are nice, the losses are not 
Alliance has some neat racials, personally I really like Shadowmeld it’s just so useful.
No nasty vulpera degenerates.
I played alliance most of BFA and the aesthetic and story were much MUCH better. Boralus vs that horrid pyramid, no contest.
pretty sure most do bcuz of the races happen to be on ally
I always played human in Warcraft 2 and 3. When WoW came out, friends wanted to be Horde, so I was there until about halfway through BC when switched over to Alliance and never looked back.