Because it culminated a year of build-up and the very occasional story The End of Time Pt I as the least Christmassy of all the specials, despite the fact that the events of the story take place in or around Christmas time. This is all the more compounded because it is the first half of a story, in this case Tennants last.
Of course the resurrection of the Master was horribly contrived, but this first-half of a story, as so many RTD two-part stories did, was compelling, and at the core of it all was the relationship between the two old souls, the Doctor and Wilf. Their scenes sparkled with the resonance of lives in full. If we were to judge both halves of this story as one for the purposes of this Christmas ranking, it would probably come out on top, but as it is it sits quite comfortably in at #3.
#2 tomorrow on Dec 24th. #1 on the day itself. All to come.