Vegard Pompey
ALLY TO GOOD, NIGHTMARE TO YOU
Haha, It's probably because it doesn't cater to any common and modern sense of viability in film. It has long scenes with no dialogue, a very strange musical score, uneven plot lines, sometimes hardly ANY plot line, an ending half-hour that would have made Syd Barrett shit his pants, and a final shot that seems to make no sense.
However, the film is highly intellectual. It's about the creation of the human race, what forces were behind it (the monolith doesn't necessarily mean "aliens;" it could mean whatever you want it to), the path evolution takes, the reason for our existence, etc.
I don't think there are any definite answers embedded in the film, or in the book for that matter. Arthur C. Clarke has stated of his novel that "If you understood what I meant by writing 2001, then I failed." Or something along those lines.
It was a work of art, and as such it was interesting. But, it bored the shit out of me, and I like movies that both entertain me and make me think.
EDIT: Or brainless gore, as long as it's really, really, really gory.