It seems kind of weird that the light would abandon the broken Draenei because of being peripherally near fel magic and energies but then fully embrace Man’ari Eredar who cynically and willingly infused themselves with it?
To quote the wiki “The foul energies that corrupted them did more than twist their bodies - it took the comfort of the Light from them so they cannot feel its warmth.”
So… how do we explain the numerous and frequent light wielding Man’ari? Seems lore breaking.
Lazy fan service
It is. If they wanted to introduce playable Man’ari they should have added it as a skin option for Draenei demon hunters if/when they eventually get added as a class.
Fel strictly speaking isn’t at odds with Light. Broken cannot wield the Light because of fel magic, but because of an altered variant of the red pox that infected and altered their looks. It’s a magical, ogre-made disease that broke them bodily, mentally and spiritually.
It could also be as simple as the Broken thinking they can’t or are unworthy to wield the Light for whatever reason and thus they cannot wield the Light.
Easy. The “frequent Light” wielding Man’ari are gameplay rather than lore.
Strictly speaking, nothing prevents the broken from using the Light. It is simply a power source, one that responds to will and conviction alone - it is not a sapient being, it cannot ‘abandon’ someone. There are even Eredar which use the Light in Legion, and I believe the vindicator boss in Hellfire Citadel does as well.
I kind of want to roll a man’ari paladin who uses the Light through sheer force of will, like they so strongly believe that they are worthy despite their corruption that it works. Sort of similar to the undead priest using the Light premise.
Broken can’t use the Light because the Red Pox was developed from the Blood of Sethe by the Shadow Council. Sethe’s blood was used long ago to cut the arrakoa off from Rukhmar’s light and destroy their ability to fly, and it was altered by the Shadow Council to cut draenei off from the Holy Light because the Horde was losing the war against the draenei.
There is nothing saying that the Light couldn’t be used by someone that uses fel. I think there is too much emphasis on magic schools and how they can or cannot interact with other schools. I know there are a few examples here and there, but as long as one feels their cause is righteous (right or wrong, see Scarlet Crusade), the Light can be used. Unless something goes horribly wrong, the Light won’t hurt a man’ari that uses it (in my opinion).
I think with undead it was established that the Light will come to those who call upon it. Someone has to have a strong conviction or will in order to make that call. It’s less that the Broken can’t because of the red pox but more that they have mentally been so broken that they believed that they /couldn’t/. It’s more of psychology based type thing.
A reformed man’ari could use the Light because they have a strong conviction or will. And therefore can call upon the Light. It may or may not hurt them. Depending on the Man’ari. Unlike with the undead, that is going to burn. And they may or may not be on fire.
Oh, right. I forgot about the Red Pox bit - good catch! Still, you’re pretty much objectively correct on the latter point. There is no mechanical reason, in-universe, that an eredar would be incapable of calling upon The Light provided force of will and conviction.
If an undead can do it, if a void elf can be a holy priest, then there is nothing stopping them. It’s the same thing as how people say things to the effect of ‘Shadow and Fel can’t co-exist’, despite Demon Hunter quite literally having multiple shadow abilities despite being predominantly fel.
Tyrant Velhari (warcraft.wiki.gg/wiki/Tyrant_Velhari) is a Sargerei/Man’ari vindicator who can use the Light - sorta.
The concept for this boss began with a simple idea: anti-paladin, or “dark vindicator.” What would a corrupted version of a draenei vindicator fight like? Many of Velhari’s abilities are a play on core Paladin mechanics such as (Un)holy Power builders and spenders, auras, and Guardians of Ancient Kings, with each of the three phases having a theme that parallels one of the Paladin class specializations.
She uses both holy fire and shadow-based attacks and abilities. I’d say she’s much akin to radical Scarlets in terms of the corrupted using the Light. With enough willpower, dedication and conviction, anyone should be capable of calling upon it regardless of affiliation.
All of her fire abilities are simply fire abilities, not entirely different from warlocks without doing the green fire quest. She doesn’t wield the Light.
They still can, though. Velhari just isn’t an example of it.
Hence why I said “sorta” - it can be left to interpretation. All things considered, she is an ex-vindicator who uses paladin weaponry; she still uses (unholy) auras which suggests her fiery attacks can be akin to Radiant damage - the same used by Ret pallies and Disc priests.
In terms of why a man’ari can be a paladin or priest, I think Velhari is a decent example. Perhaps not the best, but the closest one we can get for in-game lore justifications. It’s just a small detail for potential thought. 
TLDR we are long passed the need for lore reasons behind race/class combinations. Even more so for character customizations and class combinations.
If our character is the “special champion” They should be able to be anything they want. So lore reasons aren’t needed. Realistically though you can make a “paladin” be many things. One of my dark iron alts entirely uses fire while wearing heavy armor.
Broken are at least partially the way that they are because they were fully severed from the Light. The other part is that the “red mist” that they were attacked with in Shattrath (a form of the Red Pox/Curse of Sethe). With the Kro’kul on Argus it was more a matter of long-term exposure to the corrupted fel energies of Argus, and they’re physically different than the Broken of Draenor.
Man’ari is more of a political distinction. It’s a specific group of Eredar who took up with the Legion, and proximity to other demons and using fel turned them red over time, but there was no red mist, though, there was no severing. You can be an evil evil person and still use the Light as long as you have conviction (see the Scarlet Crusade) but with Draenei there’s a bit more intertwining of the Light with their physical bodies than the other races (even if they don’t go through the Lightforging process that kicks that up to eleven).
The Light is a part of the Man’ari still even if they chose to follow the Legion. While they take on some physical and appearance changes, it doesn’t completely warp them without additional external factors.
I’m mostly referring to people who RP Man’ari who wield the light.
so the light abandoned the Broken because of a disease, also the fel aspect is mentioned numerous times, the disease is mentioned but it’s always shown to be one of two parts, but has not abandoned the Man’ari after they willingly turn into gigantic red monsters?
Read the Wiki page, a huge part of the broken and what defines them is that they can’t use the light, it’s basically the entire reason why they pursued shamanism to begin with.
I think they just installed a Clapper on The Light. Clap on! Clap off!
Blizzard had said recently that anyone can use the Light if they have a strong enough conviction. The reason why they can’t use the Light is purely psychological. Those why they’re referred to as the Broken. The amount of trauma incurred not just from the Red Pox but also what the Burning Legion did to them took it’s toll on them. It could take possibly a millennia to recover if that’s even possible for them to do.
It’s more of a mental block than anything physical.
Like Rainaa says, mental block. I had to double check, myself, but she’s right.
They don’t dictate the lore.
The disease is responsible for twisting the Broken into their current form and severed their connection to the Light, either by literally severing it or because the Broken for one reason or another believe they are unworthy of the Light and thus they are incapable of calling on it. Plenty of lore suggests the former, whereas Light lore in general supports the latter.
The Man’ari did everything they did willingly so with enough conviction they can wield the Light, we’ve just never seen it in the lore proper.
So broken not being able to use the light was just brutually retconned to “every single broken without execption has lost all faith completely”? You can see how stupid this sounds right?
There’s no mention of this on the wiki page of it being purely psychological or mental. It’s still presented as being a choice by the light ti shun them or physiological. The only contradicting piece of info is a dev saying “anyone can wield the light” but this is pretty offhand and contradicts an entire race soley defined by being severed from the light.