I think the suggestions made above are probably all part of the truth. Another factor is that people will visit one forum more than another. If there is a new thread in the town/city mapping section, I always give it a look, at which point the urge to suggest/comment arises. I don't tend to bother so much with the regional maps (and I'm yet to visit the board game mappers). The finished maps forum, however, I visit much less often, because there is less chance I'll find something that interests me there.

By the way, if getting responses is what you are after, post something that generates discussion (mine is the 10th reply to your initial thread just today; MCC's question about subterranean city maps also generated a lot of interest in a short space of time).