Ultimately, they had to set the calendar’s dates based on something. Given the vast hold on Europe which Christianity had at the time, it’s not surprising that the starting date was based on such an important event in the mythology. However, trying to deviate too far from the currently understood order (the Julian Calendar) was likely to end in failure. So, they could either fight the tide of history or just accept a logical oddity. Given all the other logical oddities one must accept for supernatural belief, who’s going to complain about having a holy reason to eat, drink and be merry during one of the most terrible parts of the year?