As to what broke up this pop group more than 4 decades ago, the first two thoughts posted seem reasonable - combined: people change with the pasasage of time.
As to some of the others... well, let's just look at #3 "Every group eventually runs out of songs." We should remember that all four members of the Beatles kept on recording and performing, and Lennon, McCartney and Harrision continued to write best-selling songs as did Ringo Starr, always thought of as the weakest songwriter of the group.