People just wanted some other big name characters to fill in those spots. Mainly Peach, the original Toad, Wario or Waluigi.
I can understand the first two since it would make sense and be a nice throwback to SMB2, but Wario and Waluigi? Wario has his own two series now, he's hardly got a reason to piggyback off these titles anymore (and i honestly thought he was the worst character in SM64DS). Waluigi is...Waluigi, lets leave it at that.