Personally I think it has a lot to do with the fact that I feel I've seen it all before. I know it's hard to be original in your storyline, but heck, the cinematics in Diablo 2 (to me) made it's storyline more grasping than IWD2. The gameplay suffers the same fate though: make a party of six, kill weak creatures, run a few errands, kill slightly stronger creatures, level up, etc. Sure, most games do it that way, but IWD, BG & BG2 all look very similar in that aspect, by the time we get to IWD2, even with the interesting new rules and stuff, it's been done before.
Like DATD said, it's a good game still, but it's too old compared to what's available now and it repeats too much of it's predecessors.
