I suppose it's just a function of how open the previous seasons have been. I would put 1959 (and most of the 1958 races come to think of it) down to the fact that Fangio pretty much owned the previous decade, same in 2007 when we only had one world champ (Alonso) racing thanks to Schuey's half decade of dominance.

In the 1960s nobody retained their title (well except Brabham who was defending a '59 title), and only Brabham, Clark and Hill won the thing twice in that decade.