I think a mix of the first two.

Overly fierce competition
Learning curve

The early days stuff (Dune, C&C1, WC1 and 2) you could just take many many approaches, slug it out, prod for weaknesses, take your time and figure it out. Eventually though once Starcraft2 and the 2nd generation of C&C stuff (Red alert) rolled out things changed. It got very competitive on and offline for one so if you couldn't keep up or care to put 100s of hours into it you were hosed. Offline the 1P game stopped being about being smart and taking various approaches and became overly and aggressively scripted. You had to move a mass here or a mass there at this amount of time or specific moment, you had to develop this or that as the game designer wanted when they wanted or you were punished, mostly with failure, or a stalemate which fails anyway as you're stuck in a loop. That's why I stopped playing RTS at least and still go back to those old 4. Back then you could try different stuff, and they'd all work unless it was a decidedly bad idea. Now you must do what the script says or die. That is in no way fun for me in the least bit, it's a rote memory hours long waste of time, or a way to peddle $30 game guides.