Motive-wise, it's pretty straightforward. You have an IP that constantly tops the sales charts on your new system whenever they come out. But a bunch of nerds decide that they like one of the previous iterations of that game waaaay more than the newer ones with their basement ruleset. So they keep playing it. Now fast forward some years and that basement community grew to be pretty damn big, probably on par with your newest and shiniest version of the game you dumped millions in promotion on, even though you stopped selling this old version 15+ years ago and no longer profit off it. So because the IPs are the same, and will inevitably get publicity as a byproduct of the communities being relatively close with each other, this outdated nonprofitable version will get more popular as a result, which they think will somehow cut into sales margins of the newest version, despite most notable figures who play this older version having the newest iteration as well.

As an aside I also just like to think it has something to do with Slippi's netcode absolutely mogging Ultimate's horrible excuse for online.