The point is that the Wii U version's success was restricted by the success of the console it was on and Nintendo wants a greater conglomerate of people to experience the game ie. to give them monies for a game worth more than its previous commercial reception would indicate. And by throwing in a few new characters, us slaves to Smash are dragged alongside as well, giving Nintendo that sweet double/triple dipping money.
And I suppose if there was basically no break between development of 4 and 5 the cause for the port would be lessened, but that's not typically the case for Smash development, and honestly I don't think it will be here either. I obviously wouldn't turn down Smash 5, but I don't feel the need for it any time soon.
I'm fine with a port. Smash 4 was great, jam-packed with content, and will keep me and the people I play with entertained for years more to come, even if we got neither a port nor a sequel until a long way down the road. So getting a few additional characters, stages, and amalgamated content only sweetens things, especially considering the list of burning, imminent inclusions that need to make it into Smash are, at this point, really only limited to Inkling. Nintendo has its bases pretty much covered at this point.