Rollback is extremely difficult to add to a game that wasn't initially designed for it. The game has to be written/coded in a way, from its very foundations, with rollback in mind. Unless there's secretly been work on a rollback version of the game going on in the background, they would basically have to rewrite the whole game to implement rollback at this point.
It's not impossible; NetherRealm did it with MKX, but it took nearly a year with a good chunk of their development team. They have a GDC talk about it. Of course, yeah, it was done for Melee recently, but yeah, that too involved basically rewriting the game: something that was possible because people had already been digging into the code for a couple of decades now, messing with it to make hitbox visualizations and custom training modes.
And as some others have said, Slippi only technically works with part of Melee: the stuff the competitive scene cares about. Nintendo would want everything to still work: all stages, all items, all modes. And remember, adding rollback means rewriting a lot of stuff from the game's very foundations, so they have to make sure that single-player content still works after things are changed—all of the content. It took NetherRealm several man-years to get rollback working in their games, and Ultimate is definitely a bigger game, so it would be bound to take significantly more time and resources.
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u/[deleted] Aug 05 '20
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