I'm single because between full time work and grad school, I have precious few hours of the day left over for myself, and I spend those hours playing video games. If I could find a guy who was similarly busy and had similar priorities, we'd do aiight I think, but it's astonishingly difficult to meet a fellow gamer in grad school for some reason. I've only met two so far and they're both engaged. I've tried non-gamer guys and there's nothing wrong with them, but it's hard for them to understand why I would rather spend my time shooting pretend Russians or rolling dice than watching tv with them or chatting about our day or what have you.
Also I'm at the age where most guys seem to be thinking about settling down and starting a family, and I a) don't really believe in the whole marriage thing, and b) don't want kids. The last guy I dated my own age broke up with me after I told him I didn't want to get married. Since then I've been with a few quite a few years younger than me, but again, time constraints and the lack of eligible bachelors who share my priorities.
But I suspect I'll have more time for that sort of thing when I graduate.