For me there are three reasons, two of which others have mentioned.
1. Age/jaded. What enthralled me as a teenager doesn't have the same effect now. I've seen a lot of it, other stuff just falls flat.
2. Boring user manuals (the core books), that read like software manuals. In Eric Mona's thread about why people don't subscribe to Dragon, many mentioned that it's been boring, lifeless. The rulebooks read the same way.
3. Time. When I get together with friends to play, the old excitement often comes back. But we have too many other commitments. Sessions are (at best) every other week. We don't have the time to sit around and talk about gaming the way we did in college. Heck, compared to college, I hardly ever see friends, what with work, family, and other demands. All of this drains a lot of the intensity from gaming (for me, at least).
Don't take this as whining, however. Having more to life than gaming is a good thing! Just means life is full of trade-offs.