Plush, expansion fees definitely play a part. Many feel that one of the reasons for the 1990s expansions was that expansion fees would generate money to pay for the collusion mess. There've been other reasons, though. In 1961–1962, baseball was trying to fend off the Continental League. One of the main reasons for the formation of the Continental League was to put a team in New York; after the formation of the Mets in 1962, there was no longer that need. The American League also felt a need to put a new team in Washington after the Senators moved to Minnesota. In 1969 and 1977, there was political and legal pressure to replace teams that moved (Kansas City in 1969 and Seattle in 1977). I believe initial plans for the 1977 expansion called for four teams, with one of the teams probably being in Washington. But the two leagues fought over Toronto, and when the AL won that battle, the NL decided not to expand.