Or Final Fantasy, or Kirby, or Metroid, or etc.
If a sequel truly meant new material, then it wouldn't be a sequel. It would be a new game independent of the old one. Characters and continuity are part of what makes series' good, and having that stuff in a sequel is important. If Sonic 2 had all new material, you wouldn't play as Sonic, you wouldn't be in Zones or Acts, you wouldn't collect rings, you wouldn't gather the Chaos Emeralds, see where I'm going with this?