The only reason I could think of, is that he's very different on the Switch 2 version, but you can play the Switch 1 version on the NS2, so that isn't a real good reason.

Is it just because making him required would cause players to have to go through the CH4 Light World segment? If so, yeah that'll be a section with low gameplay, but you can't call it "All Bosses" and not fight all the bosses.