Because expanding audience appeal and drawing-in new players become priorities, which generally causes a game to lose depth and flavor. It is a sad (but necessary) process of mainstream gaming (and just generally part of expanding a game studio).
Also, just an observation, seems like at one point Blizzard put all their eggs in the same basket and lost track of how much money they were spending, and so WoW became their means of staying afloat between 2007 and now. This also undoubtedly played a part in the whole "gimme handouts NOW" philosophy shift of the game.
That said, I love Blizz and I will love them forever. Sometimes, it seems like they're just too far gone competing with the other big players of the industry... although, with Vivendi shareholders off their backs, they might go back to playing by their own rules. That's what Hearthstone and RoS seem to suggest.