ELI5: Why did classic RTS games die out? What was their main replacement and why did they manage to take the place of games like Warcraft 2 and Red Alert?

Ok so I know there are still RTS games being made, but there was a time when they were THE game to play.

I'm just wandering what trends in gaming caused them to die out. Was there a reason Starcraft became so huge in korea, but not anywhere near the same level anywhere else?

Many thanks