Maybe you like one class, but not the second one. Or maybe you like both classes, and you would play them, but you just don't have the time. There are some classes, that I like to play, but I just don't do raids with them, so no reason to levell them to Level 85. And even though a class might have the same theme as another class, like Death Knight and Necromancer, they would still play very differently. Or Rogue and Warlock don't have to be like Demon Hunter at all. If they just feel like already existing classes Blizzard will probably not add them. They want to add something, that doesn't feel like an already existing class. Look at the Monk.