It's a bummer there aren't more good Brian fics being written. Brian's my favorite, and I used to read and write a lot more Brian stories than I do now.
Speaking from the writer side of things, I personally don't write about Brian as much anymore for a couple of reasons: 1) I wrote so many Brian stories in my first two years of writing fanfic that I tapped out most of my Brian ideas, and 2) Brian became less malleable as a character after he got married and started a family.
I like to stick to reality as much as I can with fanfic, so unless I'm writing an AU or a story that takes place in the past, I can't just deny Leighanne's and Baylee's existence, so I either have to include them in the story (which doesn't leave many options for romance, for instance) or get rid of them somehow, which can be tough. Most of my Brian ideas these days are AU for just that reason, so I don't have to deal with the added baggage of his family. I love Leighanne and Baylee; I just don't want them as main characters in all my stories.
Kevin and Howie writers may have the same problem, but I never wrote much about them to begin with, so I can't speak to that personally. AJ and Nick still seem to provide more options because they're not so "settled." Sure, AJ's married now, but it's totally plausible to see him split from Rochelle in a fanfic, and ditto with Nick and Lauren. They can just realistically be taken in more different directions without going AU, and that may be partly why there are more new fanfics about them than Brian and the others.
