Something I've been thinking about. The twitch combat mechanics are strongly resultant of some design choices early in the genre of RPGs. What are some of the biggest traits of twitch combat mechanics? Mainly that they're quick and don't take a lot of time. Compare them to say, the encounter design of early Final Fantasy games with all the menus and turns and things. they're designed to get you in to combat quickly and get it over with. They're also designed to let you react to other mobs that join and burn down a lot of foes at once.
I think largely, this is a reaction to just having too many uninteresting mobs on the playing field. So to deal with them, you need to have methods that minimize setup, and rely more on quickness than strategy. Instead of tactics, you have combos, etc.
My hunch is this evolved from games where grind was just so boring because you'd have your encounters entirely too frequently with the same repetitive enemies. This of course was done to draw out game length, and you couldn't make the enemies too hard because it would be grueling. So you slog from place to place through fairly easy enemy encounters.
On the other hand, people never seemed to try to make encounters much more rare, but make it an actual event when you went in to combat. Each event challenging and fairly rare would be closer to the D&D boardgames that RPGs evolved from.
The common and easy to defeat enemy works decently with twitch type combat though, so maybe the evolution works.