I have a couple of previous answers that should be helpful:

In England, from 1155 to 1752, the new year started on March 25. Why?

In brief, various countries started the year on different days, and once they all switched to the Gregorian Calendar everyone agreed that the year should start on January 1, which was the beginning of the liturgical year used by the church. But why did the church year start then?

Why is January 1 the first day of the year?

And the short answer to that is "because that's what the Romans did". Why did they do that? We don't know and neither did they!