First of all, the most obvious reason, it's too complicated. The rules are so detailed there's no time for roleplaying. The combats last for way too long, and the system is restrictive and a hindrance to creativity. All of this has been stated many times before other places, so I won't go into it in detail. I guess it's a matter of taste, but for me, this gets old real fast.
But more importantly, there's something wrong with the whole feel of the game. Everything, from the way things are presented, to the names, and the artwork. I've seen the artwork hailed several places, but cannot, for the life of me understand why! Sure, the craftmanship is impeccable, but the style looks like something out of a childrens book. Absolutely atrocious!
Also, the implied setting of the book is terrible. It does not have the excellent history of D&D to draw upon, and what those guys have been able to come up with themselves is just not interesting. The names, as I mentioned before, seem like random sequences of characters. Everything is just somehow devoid of atmosphere and personality. Seems like it was made just because they needed to put something in there, not for any kind of creative joy or anything.
I know I don't have to use that setting in my game, and I don't, but still, just the feel of opening the books is uninspiring and tame. It's like playing on a cheap guitar. It's got to go!