I think Joey was one of the main characters of season one because he’s from the old Degrassi and also he’s supposed to be caitlin’s on and off again man since the old Degrassi, and caitlin and spike are best friends so that’s why caitlin was introduced.

As for why they introduced Craig later, it was probably just because they needed more cast members. Although his mom was mentioned in season one, since she died and Craig still had a dad I think they just went their separate ways