According to Masahiro Sakurai, in the November 2014 issue of GameInformer, the reason why both versions were released at different times was done to "keep the debugging schedules separate from each other:"

"The biggest reason was so we could keep the debug schedule separate from each other. Debugging a game this size is a major project involving hundreds of people, and since the two platforms offer pretty different content, we wanted to avoid conflicts during the debug stages. If we were hell-bent on a simultaneous day-and-date release, we would have wrapped up one title, put it on the back burner, and launched both games in 2015. But I doubt most people prefer that."