Everything has been said. Here's my recollection of RTSs.
There were too many and they weren't that interesting once you had played one, you played them all. There were a few neat ideas (designable tanks, stealth, emps, etc) but not many. They needed to evolve but the evolution kind of killed it. MOBA was one direction, games like Savage and whatever, they were part RTS and part fun 3rd person action game. But MOBAs evolved to become another stupid thing and there was no RTS left.
At the same time Tower Defense games came along, I really love them, mostly only the ones that let you build a maze. But that evolved too and ended up back where it started. Flagging RTS sales made them attempt games like C&C Renegade. Then Tower Defense games became stale and they ended up making 3rd person shooter hybrids too like Orcs Must Die, Sanctum, and many others. I like them too but it's not an RTS, it's not a good shooter or action game, and usually not a very good TD game either.
There were also some games that tried to be RTS without the base building. Company of Heroes, World in Conflict etc. I hated the former, liked the latter but only really because of the graphics and pretty artillery. The gameplay was kinda crap.
The nail in the coffin was EA raping and massacring as they do. Westwood Studios was awesome, they had ideas and could make stuff. I think the best RTS was Tiberian Sun and they made that. After that EA bought them and it went downhill. Generals was fun but it was less deep and interesting than TS because that's how EA does things. They did a few other ok ones, then C&C4 crashed the series into a wall.
Total Annihilation / SupCom went the exact same route. Bought by Squeenix and then they made a dumbed down version that the fans hated, The End. But try Beyond All Reason, it's really good. Based on the original TA but modified, it feels like a new game.