It's cultural. The Christmas #1 is talked about a lot, Radio 1 plays and discusses it every year, it's often been used as a way to force a message into discourse. Charts don't matter much anymore but in British culture they used to be very important to people and that lingers on in the caring about the Christmas #1. You're right in that it doesn't matter, but this one is particularly funny and I'm for putting a bunch of punks at #1 again.