Accessibility might be an issue but I think the main reason is just the same as the twitch shooter genre, there's no way to make them work on gamepads without significant concessions and automation anyone familiar with 90s-00s PC games would call oversimplification. Halo Wars are good games and probably the best RTSs on consoles for example but they're kinda pathetic compared to Ensemble classics; limited basebuilding, limited micromanagement, squads that do their own thing to some extent, and very low population caps. But basically no AAA publisher is going to fund a game that necessarily excludes consoles and PC games by and large don't want games watered down for gamepads when they're fine with KB+M, so we're left with smaller often crowdfunded projects like Tooth & Tail or 8-bit Armies.