I think that this has a lot to do with the emotions felt when thinking of the color red (fire, exciting, passion) or blue (calm, simple, soothing). It's much harder to define a brand with green (leaves? slime?), as people have become used to the above two.
From experience, it's also much easier to combine red and blue with a grayscale palette (iTunes icon, twitter to name two), which may also contribute to their popularity.
Or red (anger, blood, propaganda) or blue (cold, sad, depressing) ...we all have different personal associations with color... And add on top of that cultural.
From experience, it's also much easier to combine red and blue with a grayscale palette (iTunes icon, twitter to name two), which may also contribute to their popularity.