Why would they need more ? First generations DK are a thing of the past, there's one mention of different DK being possible is from an old Metzen interview. You don't have any obligation to provide a shit ton of flavor text or quests in order to illustrate a variety of class variations. You can show a few examples here and there. Only roleplayers care about playing a warrior that was not trained by the dude from his noob zone anyway and we can grasp any piece of information to make that happen.
Sure it was nice to have a little context for the introduction of certain race-class combinations, like the human/UD hunters, the priests/paladin tauren. But some others like BE warrior or Hunter gnomes don't need that kind of explanation.
Also you know what, class-race restrictions are what deprived us of Warlock Zandalari and Warlock/Paladin KT. For the sake of not having all classes available to a few races, they cut some combinations, while shoe-horning druids to both of them, again for the sake of I dunno, faction equity or something.