They did a new engine ground up for Brawl though. Think of it in terms of PC gaming. While Street Fighter IV and Skull Girls are both on the same programming architecture (Windows PC), you can't just copypasta Ryu into Skull Girls. They're programmed completely different.
Not putting him in because it was more work however is kind of weak. They're Nintendo, they have the bank to do it.
Cheap is exactly the word I use to describe this Smash Bros honestly. No elaborate adventure mode, FMV's, etc. They made this game on the cheap compared to Brawl's massive production values. Given Brawl's budget, which was a ground up new engine so every fighter had to be recreated, extensive story and adventure mode, crazy amount of characters, etc, you can start to understand how much of a budget title this was in comparison. The more I see of it, the more it is indeed underwhelming.
It DOES play better than Brawl, so I'll give it that. It also has For Glory mode, which is a huge plus. However, I still feel underwhelmed by the huge lack of the single player offerings, especially when compared to Brawl. It's really regressed back to N64's single player, but with even less honestly. Not even any bonus rounds that I saw.