Giles Fraser, late of St. Paul’s Cathedral, had a radio program on the BBC on Christmas Day in which he argued that:

Following his battlefield conversion, Constantine established Christianity as the official religion of Rome, and he decided that Christ’s birth should become a major focus of the Christian year.

The broadcast is available here. But for a better historical perspective, try Andrew McGowan’s piece in Biblical Archaeology Review. It’s much better history, and much better nuanced, making clear that it wasn’t until the mid-4th century that December 25 became the accepted date for Christmas, and pointing out that there was considerable speculation about Jesus’ birth as early as the gospels, and increasing in the second century. He even goes a long way toward debunking the myth that December 25 was deliberately chosen to counteract pagan rites.