[corry]

I think it's simpler than that:

(1) People learn to play guitar by playing popular songs. These popular songs feature a subset of chords in certain combinations that sound good (eg. C->G->Am->F, C->Am-F-G, Am->F->C->G).

(2) People start creating their own songs using the mental maps of "what chords sound good together" from step #1.

(3) Rinse, repeat

My feeling is that it helps that these patterns are easier to play in C, but I think it's really about matching the "what chord comes next" pattern to a particular key.