According to the Smash Bros. director, only one developer was suitable for the project.
http://www.nintendoworldreport.com/news/39323/sakurai-talks-smash-bros-development-roster-creation-more
Super Smash Bros. director Masahiro Sakurai recently shared a few new details regarding the development of Nintendo's popular fighting game for 3DS and Wii U.
In an interview with Nintendo Dream, Sakurai said that finding a developer to handle the latest entry in the series was a no brainer, as Bandai Namco was really the only team capable of handling such a project. "We thought it would be better to have one single company work for us, so we choose to work with Bandai Namco Games who produce Tekken and other games," he explained. "When thinking about a Japanese company that can make a large-scale fighting-game, there was only this one company. It would be quite difficult for other companies I think."
Additionally, Sakurai revealed that the game's final roster was nailed down all the way back in May of 2012, before the Pokémon Greninja's name had even been decided. "For example Greninja, even before his name was decided I received several illustrations. I took them home in the evening and around midnight I had already done all his actions, normal moves, special moves and pose-pictures and sent them around asking "What do you think?"."
The fourth installment in the Super Smash Bros. series launched last year for Nintendo 3DS and Wii U. You can read our review of the 3DS version here and the Wii U version here.