It's impossible to deny the instantly recognizable archetype of the demon hunter, that much is certainly true. The question is, how do we make their game play compellingly different from a warlock, or a rogue, or a monk's, without gutting those classes of abilities they've grown to love and rely upon? The most obvious and egregious offense would be to take all the demon-hunter themed abilities from warlock and give them to demon hunters.
Now, I'm not completely against demon hunters. Not at all - I think they could have been implemented as a class that's unique and compelling as it's own right - but all the niches they could have filled in this manner have been filled since. Death Knight's have provided a melee DPS class that relies heavily upon diseases, control, and spells, similar to how I imagine a demon hunter would be in a DPS spec. Replace diseases with burns, magical dots. Their tanking niche has been dominated by monks, and to a lesser extent druids. Evasion tanking, using damage reduction to shrug off blows they should not be able to survive.
Honestly, the lore reasons are the weakest of the reasons I gave - except that now Demon Hunters feel more like a subset of warlocks than they do their own class. Frankly, I'd be overjoyed if we say 4th lock spec that was literally a demon hunter tank spec - and a tinker class in the next xpac.
But that last bit is just my opinion :P