Because Blizzard’s target audience is dumber than what it used to be 5 years ago.
You can’t expect to give players choices (you know, like in an actual RPG) and see them do well, at least not the target audience that Blizzard has in mind. You need to hold their hands and give them pre-programmed characters to play instead of actual classes, so you only need to put up a mask of it being an RPG even though it isn’t.