A racing game like Mario Kart 8 feels more defined by its courses whereas Smash is defined more by its characters and the unique movesets they have. It sucks stages are going to be released at such a slow pace, but I can see why there's a difference in priority for what gets released when you compare Smash to MK.
I see why a lot of people feel this way, and for other fighters I completely agree, but for me smash has always been heavily defined by both it's characters *and* stages.
Every layout is incredibly different much like Mario kart's tracks and much like a track they define the course of the match. Having a stage roster be very smelly severely limits the sort of games you can play.
The different versions of hyrule castle and peachs castle are vastly different, same with battlefield and dreamland 64.
Honestly I'm really happy with the character roster, it doesn't need much more imo but the stage roster is pitiful compared to brawls letdown of an offering.
*especially* since if you were to take both versions of smash 4's stage rosters together you would get a really unique and well-rounded group of stages, but both feel lacking when they're cut in half the way they are.
A $20 pack of 3ds stages and a $20 pack of retro stages would sell like crazy and add SO MUCH to the game.
Games like street fighter where the stages are merely backgrounds don't need that many, but if the stages significantly change, or even define the match then having a stage roster that smells is equally bad as having a bad character roster imo, though the latter is far more apparent