Several reasons. Family from out of town, people like to come as a family for Christmas, Chreaster Christians that just show up twice a year and that's in addition to the usual folks that always show up for Mass. I used to resent the twice a year churchgoers but Christ would not quench the smoldering wick, nor would I.
A priest, a minister, and a rabbi are sitting around drinking coffee and complaining. The minister says, "I can't get rid of the bats in the steeple. I've tried everything and they just keep coming back."
"Oy!" chimes in the rabbi. "They're under the eaves at the synagogue, I've tried everything - exterminators, owl decoys, wind chimes - nothing helps!"
"Hey, no problem," says the priest. "I just baptized and confirmed them, and now they only show up at Christmas and Easter."
I personally think those Chreaster Christians will be stting this Christmas out -- unless they can find a service in the evening or late in the afternoon. This is the first Sunday Christmas in over a decade. My suspicion is that if they are not inclined to go every Sunday, then they are certainly not going to be inclined to get up on Christmas morning and go to Church. My bet is that they will all show up at the Saturday Night service. Better break out the folding chairs for that one.