[pge]

I think you are misunderstanding the article - All the songs were transposed to the key of C for the purpose of analyzing chords, so don't draw conclusions from the actual chord names presented as the most populat. Instead, what the article is saying is that the I, IV, V chords are the three most common (as one would expect from basic music theory). In the key of C, these are C, F, and G, so that is how it is presented.