Maybe NX is not backward compatible?Have you thought about this?
Not only that, but Smash 4 has alot of DLC.They could very well just a want a re-release with all the dlc in one package for the NX, even if it has backward compability.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1