Skullgirls definitely set the difficulty to Insane before hitting Start, but even then, that's not why the game isn't bigger. If Lab Zero hadn't run into all the troubles they've overcome, Skullgirls would have been a game that received constant updates and new characters on a somewhat regular basis, like it does now. The difference is, the momentum from being a new game would have meant something. The long delay between when the game first came out and when they were finally able to add new content to it is what hurt Skullgirls the most.
The reason why BB, and Arc Sys games in general, aren't bigger is because Arc Sys doesn't care about making them bigger. Or rather, Arc Sys doesn't care about anything besides Japan. So rest of the world gets ports late, the hardcore that are the main driving force in building and sustaining a community have limited interest because shit's old, and likely soon to be outdated knowing Arc Sys. Even though BB is fairly popular with casuals, there's no highly motivated scene to draw in those casuals.
The other really important thing, and probably the most important thing, is that the companies releasing new fighters are smaller and aren't capable of throwing major marketing money behind their games, which means they're not going to blow up quickly, even when they've got the gameplay, the characters, the setting, and the story to succeed.