A good reason I can think of is balance concerns. A lot of the new mons from S/V are really, really strong Pokemon. New game altering abilities, new game busting moves with high BP, min/maxed base stat combos, potentially setting breaking designs --it will probably be more work to integrate Gen 9 Pokemon into the game than the Gen 8 Pokemon, considering the difference in power level between even weaker Gen 9 Pokemon and rest of Rejuvenation's roster. Players wouldn't have access to most of the new mons until relatively late, lest the early game be made trivial, and editing enemy rosters to incorporate new mons will take a more fine touch than the Gen 8 roster updates, which were themselves hard to do too, as I understand. I don't see current Rejuvenation being improved with the S/V additions. Maybe including S/V concepts as a central part of the post game would work well enough to justify including them.