Because Melee and Brawl are entirely different codes on entirely different engines, so direct conversion is nearly impossible or extremely tedious.
People have also already gotten the models from the game, and P:M is not Melee 2.0 anymore, so...yeah.
Besides that, that'd be...well, it wouldn't help the whole legalness of P:M situation for Nintendo.
Direct use of assets you didn't create without any modifications, especially from the PMDT, would be the straw that snapped the camel's back, as it were. That sort of shenanigan would result in Nintendo taking action against it forcefully, since Nintendo owns those assets.
Roy and Mewtwo in P:M aren't at that risk because the models there are entirely custom-made, not ripped from intellectual property they don't own.