I do think one answer that is semi legitimate is this - churches can often become Christian ghettos, in the sense that most of the Christians you are in regular contact with (particularly if you are involved in local ministry) are probably at your church, and unless you're in a large church, that can be a small pool of people of an similar age in total, and perhaps an even smaller pool that would date you.
Another thing I think of is this - as much as we should only really date people who hit the baseline facts of potential marriage - (opposite sex, Christian, not a complete psychopath), I do wonder whether we don't overly conflate dating with courtship/engagement/marriage. Why, particularly in evangelical culture, is it increasingly uncommon (if not outright unacceptable) for people to 'date' other Christians without it being essentially and immediately paving the road to potential marriage with that particular person?
Is it acceptable to go out on a date with someone without instantly being in a serious relationship with that person. Is it possible for Christians to go on dates with multiple people in a relatively short period of time, conducting in all godliness, but not assuming by any one of those dates that one is in an immediate situation. Is a requirement for a date actually "I could see myself spending the rest of my life with this particular person?" When people reply with "I don't really see us being in a relationship", does that come out of a biblical perspective on dating, or is that a part of our Christian 'culture' of what dating is?
Perhaps that is all a little off topic and should be in its own thread, but it's something I've been wondering about.
To steer on topic
, perhaps what I honestly want to say to people that ask is, "Because I often wonder whether we've made a church culture that makes it very difficult for people to get a sense of what a relationship is, and whether they want to be single or not, and understand what each involves?"
Sometimes I also want to say, "Because I'm terrible with women"
Or, "Issues"
But usually I shrug my soldiers, put on a half funny, half apologetic grin, and say something about how I'm too busy for that kind of thing (which is semi-true as well, I suppose).