It all boils down to an actor wanting control over their likeness. If it was a bad performance, you wouldn't want people thinking that was you?

If not for the bad blood between Glover and the filmmakers, things might have been ironed out in advance. His replacement didn't get a large credit either.

I don't know what was said when BTTF II came out, but the filmmakers certainly were not drawing attention to the Glover situation, at the same time not making a secret of reshooting the ending of BTTF, to incorporate the new Jennifer.

Nobody was thinking sequel when the original film wrapped.