Then why is it so easy to find ranked matches in games like Skullgirls and Killer Instinct? Those are both relatively niche compared to the heavy hitter franchises. Is GG really that much smaller in terms of player base?
Depends when you are playing and, matching with someone "easily" doesn't mean you two are around equal level. I don't play Killer Instinct, but this is the experience I've had with smaller fighting games.
I mean even in street fighter 5 depending on when I play I get some shitty ranked matches.