"Suddenly" is a bit of an exaggeration. The Christian faith was decriminalized with the Edict of Milan on 313. It wouldn't become the official state religion until 380 (as you note later), long after Constantine's death. Such a change, from a persecuted faith to official state religion would be impossible to enact in one fell swoop. Perhaps saying that suddenly Christianity became an accepted religion and paganism began to decline would be more accurate.