Charlie Anders seeks to answer that very question:
(Click on the graph above to go to the full size original graph, which graphs the frequency of stories per season in which the Doctor is portrayed as overthrowing the government or the status quo or foments a rebellion.)
Although the arch-conservatism of the John Pertwee-era Doctor (which continued into the early part of the Tom Baker-era Doctor) is not that surprising (at the time the Doctor did work for the quasi-military U.N.I.T. as its science advisor, after all), who’d have thought that the Sylvester McCoy would be the Che Guevara of Doctors or that the current Doctor would be relatively tame, given that he is a Time Lord without a home world?