The reason that Saturn/Dreamcasts are on the VC is simple - Space.
Saturn used CD that hold 700mb (I think), while the Wii's Internal memory holds 512mb (again I think)
and don't even consider DC as it used GDROMs which are even bigger and some games like Skies of Arcadia came on multiple discs.
I expect Saturn and dreamcast for VC for the Wii's successor though








