The core reason is this:
There's a huge part of the playerbase that wants a Demon Hunter.
Everything else doesn't really matter. You are all playing a game where, next patch, "Turn Undead" will be able to turn a priest or paladin, but not a Death Knight's ghoul. So, they'll bend it to be cool- they believe the Rule of Cool trumps any manner of verisimilitude.
Which, in this case, is probably ok.
Here's my quick list:
> The Burning Legion remains as a standing enemy, and has been hinted at in current tier. Demon Hunters would make a great class for such an expansion.
> The playerbase has been spamrequesting DHs since vanilla alpha.
> The WoW game needs more iconic Warcraft stuff. The game has had a lot of generic classes running around for a game with such a distinctive IP collection.
> The Demon Hunter is generic enough in his specific abilities that a variety of playstyles could be packed in, if the flavor was correct.
Thus far, in game, Demon Hunters have been either Night Elves or Blood Elves. They could keep this restriction, or add in some races- at the very least, allowing humans and orcs seems likely, and they could probably fill the roster pretty quickly with almost all the races, as they have for all classes except for druids, alliance shaman, and horde paladins.
The game has 3 on every armor type but mail, so you could add the Demon Hunters as a mail wearing class. Monks and Demon Hunters are unarmored heroes, so you could meaningfully put them in anything but plate, and the money is going to be that they are an agi mail spec in at least one place.
The Demon Hunters we saw were beatyface guys, but that doesn't exclude them as tanks. They had a couple ranged powers but were mostly melee, so they would need at least one melee spec. You could argue that they should be:
-Pure Melee like rogues.
-Tank hybrid like warriors and DKs (melee/melee/tank)
-Unique hybrid (melee/ranged/tank)
-Tank/healer hybrid like monks and paladins (melee/healer/tank)
The last one I don't think we'll see for them- healing powers would be a stretch. The devs are frustrated with pure classes, and even occasionally range about trying to deal with Frost/Unholy and Arms/Fury. So, I think we'll see:
Agi mail melee
Agi mail tank
Int mail ranged caster
You could also just do it all with Agi mail, but for whatever reason the devs have been very defensive about letting anyone get close to the hunter's niche- agi mail, ranged weapon- even without factoring the pet in. If it used different gear types, it would also be harder to swap back and forth between your ranged and your melee spec.
I think the above is likely for those reasons. Also, no one else in game can run ranged and melee in the same gear, and I think they like that. Allowing agi mail to be used for melee, tank, and ranged would likely be too much- I think if they didn't want Int gear in the class, they would go with two agi mail melee specs and one agi mail tank spec.
The thing where you become a demon and wail out damage would probably be a defining point of the spec. Like the warlock reimagining of the demon hunter original, you would want demon form spells that replace your normal spells. It would be a hard sell to get this correct, with demonology locks already basically having the fully implemented version of this, but that just means they would need ways to differentiate the methods, just as windwalker is very different from combat, despite sharing weapons and armor and energy and (renamed for the monk) combo points.
If the spec takes the path of having 2-3 melee specs, then you would presumably want one that is more focused on dual wielding, one that is more focused on demon form, and a possible third spec kind of escapes me. Much like DKs, you could go ahead and assume that they have access to types of magic- blood magic, frost magic, and unholy magic for DKs made for three pretty different damage types at LK launch, and if DHs have access to fel energies, fire magic, and physical prowess it could map to that.
If the class only has 1 melee dps spec, then it would presumably be about the weapon mastery outside of demon form, and burst damage inside of demon form.
Tank wise, there's some room, but overall tanks are packed kind of tight. We already have a tank who heals themselves with dark magic, and the stagger mechanic is very much A New Thing in mmo tanking- other games haven't even got around to copying it yet, it's that new! One of the problems with tanking is that you can't really have the "demon form / normal form" thing mean the same thing as it does for dps- either the transform would be mostly cosmetic for the tank, or it would remain a dps cooldown without much or any defensive benefit. This isn't to say thay can't fit it in if they want another tank, but it's not as easy as it was for monks- and they definitely pulled out the creative lotion to make Brewmaster feel new and different.
Ranged dps wise, it's clear that you'd have to be careful to avoid having the demon hunter step into warlock territory thematically. It would be interesting if the spec had to build up a demonic energy by being in melee (presumably with attendant conversions from Int -> Agi, if, in fact, the spec uses Int mail), and could then spend it at range as a demon. I think they have the greatest leeway to come up with something here- as long as he doesn't straight up spamcast like casters or lean on a bow nonstop like a hunter, he has a lot of room to do cool things. Alternatively, the demon hunter could melee from range with magic melee hits (like enhancement shaman) to build up their ability to transform.
So, that's my thinking. Everyone loves demon hunters!