I think the competitive gaming aspect had become very stratified. I was nasty Undead player when I was 15 playing Warcraft 3, but to get there I had to pump a lot of hours in, and hours I knew I'd suck in. There was no reward mentally, I was either crushing a worse player or getting handled bya better one. The people still playing RTS game online are increasingly the latter. In a game like Overwatch, I, a run of the mill Gold player, am gonna get wrecked here and there. But I'll have my moments, my ults and point holds and stuff, that will keep me thinking I'm pretty good at this game.
I also don't think people really feel like playing extended campaigns through the medium of real-time strategy. That many of these feel tacked on and not Brood War quality is another problem. It's a genre you get actively worse at as you age, and I feel the generation replacing us never really had that game that showed them stuff like micromanaging, scouting, and unit counters can be fun.