I've actually been saying this to a lot of my friends for a while, and I've gotten a bit of slack for it but I'll say it here: I really think the development for Ultimate was cut short just for the sake of Nintendo wanting to sell us an $85 game. Like, think about it: with the Grinch leak, we were expecting to see Ken, Chorus Kids, Banjo, Geno, Mach Rider, Isaac, and Shadow. 7 newcomers (sure, two echos, but I digress) that we EXPECTED to see in the November direct, and we only got 2 (not including the plant). And after the leak, everyone considered the potential roster to be amazing, but also very fitting for the general theme of the "ultimate" Smash Bros game; a LOT of new stuff.
We expected 7, and got 2, and an announcement that 5 would be sold to us throughout the year. I really do think that there was either WAY too much focus and expected hype behind bringing back every legacy fighter, or Nintendo just really wanted to sell the game for more than $60. Like, don't get me wrong, I'm still absolutely ecstatic for the game we got, I think the roster is amazing the way it is and the mere existence of the game is enough to make me very happy. But still, I'll openly call a spade a spade and admit that I do truly believe that Nintendo really wanted to get the most money out of each buyer that they possibly could.
With that said, I do think this trickled to other aspects of development as well. The game does objectively have more content in it than any other Smash game ever released, but completely abandons the idea Smash Run or Smash Tour, disregards trophies and exchanges them with JPEGs, thinks preferred rules that only work maybe 10% of the time are a valid replacement for For Glory despite easily being able to implement both methods, things like that. And this surely affected the ability to add in more stages. I am honestly pretty irked that the battle theme for Sun/Moon was thrown into Incineroar's reveal, but not a new stage. You're right, Alola Pokemon League would've been a perfect stage. Damn shame, really.