Now, granted I do recall SC6's matchmaking being notably slow even when the game first came out... but I don't necessarily think that's the issue here. It's a fighting game that's almost two years old and is neither one of the big, popular ones (Street Fighter, Tekken, Mortal Kombat, Smash Brothers), nor is it a licensed game based on a very popular franchise (such as Dragon Ball FighterZ). The daily active player base has probably just dwindled that much at this point, so finding games on ranked is no longer very practical, as most of the few remaining players just stick to lobbies. Happens to most fighting games at some point, and unless you're one of those big names, typically a couple of years after release is enough to do it.
I'd just forgotten about how much of an issue that was likely to be in this case, since I've spent the past couple of years either playing games that are relatively insulated from that issue (Smash and DBFZ), or one where the lobby system is better and doesn't induce those annoying wait times while other people play (BlazBlue Cross Tag).