In a recent interview with Nintendo Life, Sakurai-san explains the process in which 3rd party characters are chosen for The Smash Bros. series.
“First off, I think about which third-party characters would make suitable guests for the game before beginning any negotiations,” Sakurai explains. “My priority is to provide the best experience possible to consumers and so I want to make as much as I possibly can for them.”
I highly recommend checking out the interview, as he discusses how much time went into each character, who he personally plays with, and insight into other additions to the recent iteration of the series. Go have a read!