With multiplayer games people can play the same title forever if it's deep enough. So new games in the same genre have to compete against a game that was made by Blizzard in 2010 and constantly developed ever since.
So what happens is - people check out the new game, and either get bored (it's too similar and worse) or get discouraged (it's too different to bother learning).
The only remaining market for RTS are so called "casuals" that want single player experience. But they don't engage with the game enough to justify years of gameplay development, balancing, innovative mechanics - they mostly want nice cutscenes and good graphics and beliveable dialogs and plot.
And most of them won't even finish the game. They will play for a week and move on. Single player games that don't achieve cult status - are finished on the market after the first month.
So for a new RTS to compete it has to be better than Starcraft 2 in multiplayer and gameplay or at least in storytelling. And that's a VERY hard thing to do for an indie studio. Risk is huge and required investment is enormous.
I've had high hopes for Iron Harvest and it clearly went for the casual/single player market and did pretty well, but not well enough.