A theologian I spoke to pointed out that even Martin Luther didn’t revert the changes Constantine wrought, like the holy day being on the sabbath instead of Sunday to commemorate Easter every week. I guess roughly 1200 years of habits are hard to break, when Constantine only had a few hundred years to contend with, and there were so many sects of a very different Christianity even then (Nestorians, Syriacs, Copts etc).