I simply cannot see how people prefer it over the turn-based combat system.
Because RTwP is fun.
RTwP is faster paced.
I mean, if you're playing on some low difficulty setting where you don't need to micro-manage everyone for any 0.3s period you can steamroll any combat encounter in turn based as well, it won't take long because you don't need to think about anything, I promise you that. Personally I didn't mind my characters running into fireball as much as I did mind them not doing anything without 10000 hours of AI setting up (without the AI setting up they'd just waste spell slots casting magic missles at basic 1 hp rats)