As coffee n cocoa has pointed out earlier some pairings have fics because one or two people write them and keep their work posted on the comm. I write a lot of Neville and Hannah and post them frequently, but sometimes I feel I'm the only one writing it. I have a hard time finding more than a handful of stories that I haven't written myself or asked for. (The same goes for Alice and Frank.) If I want to read a fic that I haven't written then I have a hard time finding some I haven't already read. To me that makes the pairing rare.

(I'd be happy to accept it if they are deemed not rare.) So I feel that if the pairing appears to have a lot of stories, but really all of them are written by one or two people that makes the pairing rare. (This is my humble opinion only.)