I disagree with this premise. I think games like age of empires and StarCraft had mass appeal and success. They brought in audiences who don't normally like games, and broadly were well received by young, old, and different genders. Especially age of empires 2.
Modern RTS games are just (mostly) sloppy, unfinished, cashgrabs with no vision. They suffer the most from the transition to 3d as well. If a major studio actually put work and time into a polished, 2d, isometric, RTS that wasn't solely focused on being an esport, I think there is a major vacuum for them to fill.