I think that while popularity is a factor, mew fics get replies as well (mr_pikachu).
For one thing, most fics are popular because they're good and have been keeping up to standard for quite a while. Titles matter as well, and originality. I mean, the only reason I go to stuff like "Amy's Quest" (no offense to anyone whoes fic is actually named that) is to give them a few pointers, assuming they're newbies. But most of the time I'm surprised by the stuff that's inside. But then, not everyone criticises for the heck of it, so better titles are still good.
Do some nice blatant advertising in the Fanfic Updates and Summaries... and the rest is just waiting. As your fic goes on, more people will catch on (unless you don't update for two months, like me.)