Why do some “not so great” fighting games have large active communities while others don’t?
It comes down to network effects. To put it simply it’s often better to play a fighting game A (which might not be your favorite) with good competition over fighting game B (which is your favorite) with no competition.
Hence many not so great fighters will be adopted by the community for various reasons. Perhaps it has great graphics that attract many people (you can’t deny graphics can break a game -e.g. CFE was great but had shitty graphics). Maybe low level gameplay was “fun,” and many people played, then high level play was discovered via some glitch or tactic. The game ultimately came down to that glitch or tactic. At times the top players of the community may support a game (that might not be great), and that gets many people aboard. These are just some possibilities - not to mention any history.
IMO the easiest fix for this would be 1-2 frame lag netplay with community support. GGPO created a VS community out of almost nothing. The interface of GGPO and finding matches is greatly superior to typical p2p frameworks. However with the rise of nFBA (and hopefully better community support in software), it is possible to play games like JoJo, SS2, random SNK game here, etc.