Because Christmas is Christmas. The shops are shut, the family tends to gather round the telly for events like this. It’s the big show for the year, and Who has been at that centre since it came back.
New Years is not remotely comparable...shops are open, it’s seen as a time for friends not family, people are more likely to be back at work even. The celebration itself is the night before, the day after is...meh. A bank holiday like any other. There is no sense of such an episode being particularly special in the way a Christmas episode is. Some shows will even just have their normal episodes on (proper New Years Specials are on New Years Eve for start.)
Whatever it’s viewing figures are, they would have been higher at Christmas. A Christmas special is an event. A New Years special is not. It’s a downgrading in Who’s status.