Of course this is just my understanding on the matter and I could be completely wrong, but wouldn't it be keeping with his justice if he let the whole human race stay under condemnation at the fall?
As I understand it Abraham was not ac worshipper of Jehovah but an Idolater when Jehovah imputed righteouness to him for no good reason except Jehovah wanted to. So now Abraham's faith would be the vehichcle God would use to impute righteousness to others.