Mostly because at the time of the original release people were sick of the "New" series and art style. It was the 4th in 6 years and only 3-4 months after the 3DS release. People wanted Nintendo to mix things up more.
Except we're far away from that, now. Shouldn't we take a step back and judge a game based solely on its merits instead of comparing it to our expectations from more than half a decade ago?
Super Mario World is one of my all-time favorite games (it's in my top ten of all time) and even I have to admit that NSMBU is, from a gameplay and polish perspective, almost as good. It will NEVER have the nostalgia or aura of mystery around it like SMW, and the soundtrack is sort of bland by comparison but from the standpoint of design and world building and gameplay? It's on par with the best.
I got it all, baby!
PS4, Switch, WiiU, XBO, PC
Vita, 3DS, Android
Top 3 this generation:
Bloodborne, The Legend of Zelda: Breath of the Wild, Dark Souls III