I think part of it is the corporate nature of the owners of D&D now, and part of it is that the D&D 1st and 2nd editions and to a lesser extent the 3rd edition, were designed to appeal to the sensibilities and tastes of the Dave Arnesons and Gary Gygaxes of the world, i.e., middle-class western European or North American, mostly white and mostly male nerds.

The later editions have much more general audiences from much different generations than those early ones, and so the game changes - and that's not a bad thing. Buuuut, combine that with the impetus of a corporation to make their product sanitary and palatable to that wide range of tastes, you end up with something bland and inoffensive.

edit: none of that is to say that I think 5ed is a bad game, it's not, it's just different in kind and in philosophy to the earlier editions.