Because it's shallow, and I don't care about the characters or their motivations, nor is there any real attempt to make me care. It's full of itself, over how many homages it can fit in, how many "look at this!" moments. Wow, attractive women do martial arts and fight with swords! Yawn.
The entire film, including vol. 2, is an overlong ego trip. It's an experiment in just how masturbory a piece of film can be. Vol. 2 is better though, for at least slowing down a bit and giving some time for a few nice moments between characters (though not without some truly annoying geek moments, like where Bill all of a sudden has to go on an unneeded talk about comic books).
Overall though, total overrated shit. I can say the same thing about Tarantino in general, actually. If I have to hear one more fanboy say Pulp Fiction is better than Shawshank or Schindler's List or <insert better movie here>, I'm going to lose it.
(Not that The Punisher or other films have crazy depth to them, but those films aren't trying for Academy Awards, like Quentin wants).