Because every other aspect of the game is basically the same?
Like cut the crap. When not-Red saves Bill from a teleportation mishap, battles on the S.S. Anne, calms Marowak's angry ghost, stops Team Rocket from taking over Silph Co., etc, do you still think that's a "new character"?
Something to note is that the dude who leaked the titles and concept of these games mentioned that while Red and Blue are in the game, that you don't play as them.
With most of the story beats being the same, makes me wonder if Ultra Space is involved with Red makinn an appearance, and we're looking at yet another alternate universe, this time of Kanto. Maybe the new Pokemon is an Ultra Beast too that leads to this meeting!