It started the 'downfall' ages ago, it was just that only a few people saw it. The same can be said about the PvP community in general nowadays. MCSG and Minecraft PvP in general have both lost their spark.
It's because there isn't that much community interest in the game now compared to a year or 2 years ago. All the things that there are to be done have been done, and in great amounts of. The struggle nowadays is to create something unique, but there will almost always be some type of gamemode that is in some way or another similar to your idea.
There's also less interest in current gamemodes because they've been around for so damn long that it gets repetitive after a while, or just not fun at all. Try playing the same song on a musical instrument everyday for 3 years and you'll know what I mean. Eventually, you do reach a point where it's fun because you know it so well, but after that, the familiar gradually transitions to the boring. Some people still manage to make something out of these older gamemodes, something that they still can get from it- but there's only so much of that that can happen too.
UHC is probably one of the only 'avenues' of Minecraft PvP that haven't exactly gotten old yet, simply because its time in the limelight is quite short in comparison to things like MCSG.
tl;dr the game is aging, and along with that comes lack of interest within the community