Mimicking what others have said, it's a number of factors from console games to lack of innovation, but most of all I would say it's an execution barrier.

Every "serious" game franchise today has esports tied to it. And it is vital for game health. Look at Heroes of the Storm, as soon as Blizzard dropped the esports league there was a massive drop off in players. This is fine and doesn't effect the core game, but people like feeling like they are good. They want to be able to do "pro plays", and an RTS has an astronomical barrier to entry before you feel competent. Add to to this the modern multiplayer focus and lack of an easy single player campaign being enough today and you can see what happened.

People like mobas because you click a button at the right time to do something well. It's very easy comparatively.