At the time it was issued the Church was in its infancy and was hardly the powerful force it would become. Constantine (and, indeed, many early Christians) never completely abandoned Paganism. The Church was not born a homogenous nor influential entity. It only began to become one because of Constantine.
In the century after Constantine, Leo the Great complained about people doing devotions to the sun on the steps of St. Peter's, while on their way to Mass. Syncretism of solar worship and Cristianity obviously went on, to a significant extent, and at that time the
church was unable to stop it. There was a lot of official syncretism, which was never admitted, and is often ignored today.
Christmas is an obvious example; those of us who speak English name Easter after a pagan goddess, and so on. But private syncretism was never approved, at least in 'ordinary' people. I'm convinced Constantine combined sun worship with Christianity, and it was covered up.