Played one of these the other day.
Runs at like 27 fps when I normally get 200+ in Sc2.
Without having studied the subject all too much I'd say the answer is a combination of MOBAs, F2P, Korean Sc2 dominance and certainly other factors as well.
At this point I'd be happy if we even get a third SC core RTS. A fourth Warcraft one however; I dare not even hope for that.
SC2? No.
SC1? Yes. Koreans dominated SC1 for a long time and SC2 was fairly neglected for a decent amount of time when it came out from Korea. It wasn't until 2012 or so when Korea started to push out, and by then League of Legends was taking off as well as DotA2 and every other game.
Let's look at PC games around the dominance of RTS:
RTS games you had C&C, WarCraft, 2/3, StarCraft: Brood War, Age of Empires, Homeworld, etc.
Shooters were evolving, but you had CoD1, CoD2, Medal of Honor, Unreal Tourney, CounterStrike, Half Life. (Most of these were very slow paced games still)
RPGs, you had stuff like Morrowind coming into play, you had the Baldur's Gates, etc.
So look at all these games and how most PC games were relatively slow. Slow but very intense in requiring strategy and awareness.
Compare to games now, where you have fast paced shooters such as CoD, Titanfall, BFH, etc, with the MOBA genre one could argue is faster paced (and arguably simpler to play as you focus on a minimap and 5-10 champs).
Now look at games like Planetary Annihilation, StarCraft 2, really the only two RTS that can be claimed to be "alive" games. They're fast paced, but very complicated, strategical, with steep learning curves.
Why would I spend my time learning that when I could install CSGO and shoot someone in the face almost instantaneously?
One last thing would be microtransactions, proper microtransactions. What gives a player an incentive to keep playing? Jealousy and investments. In league of legends, dota, csgo, you can spend money to buy skins (still worth value after purchase in the latter two) and cosmetics that fit the game more to what you like to do.
In Planetary Annihilation, all units look the same, blocky and colored. You can get new Commander bots for the absurd price of some $15 despite you being a EA backer.
In StarCraft 2, there is 0 monetization or incentive to play the game past ladder.