Not everyone can micromanage. More than 2 characters at once, that is.
Yes this is what it really comes down to, but people don't want to own up to it, for reasons of pride or ego.
I'm not trying to insult anyone. We all have things we're good at and things we're not good at. I myself am limited in my ability to handle games that require a lot of hand-eye coordination because I did not grow up playing video games, have never played console games, and as such trying to play with mouse and keyboard a game that requires a lot of hand-eye coordination is extremely difficult. So the argument that RTwP games require too much micromanaging and multitasking that is difficult for some people I can appreciate. All other arguments are BS.