Not that you asked me, but here's one reason: I have keratoconus and even with glasses and multiple eye surgeries don't see 20-20. Moreover, my right eye is considerably worse than my left, which means that I can't quite get binocular vision to work, making 3d viewing impossible for me. I realize folks like me are a tiny minority, however.

Perhaps a broader reason is that 3d seems to encourage filmmakers to go for visual spectacle over good storytelling.