The Dodgers and Giants moved to the west coast for the 1958 season. In 1957, there were three MLB teams in New York; the Yankees, Dodgers, and Giants. In 1957, there was one team that played west of the Mississippi River; the St. Louis Cardinals. Lots of open territory meant lots of fans and lots of profits. With air transportation becoming more the norm, making it easy for teams to travel to and from the west coast, it only made sense to leave an area with three teams within a few miles to go to an area with zero teams within 1000+ miles.