A producer and director have to make choices. Obviously, the theatrical release of I&II have far less material than the Saga, etc. So, even though the Saga et al are quite enjoyable, they are a luxury that theatrical releases do not enjoy. Both films were 4 hours. That's quite alot for an audience to bear. To add more film increases the time of the film and the studio doesn't know how well audiences will tolerate that.

I think that Genco's relationship with Vito was adequately explained in GFII. GFI was not retrospective like GFII. Genco was on his deathbed in GFI, so there was nothing contemporary to explore.