As it stands now, Sunday (although to a lesser extent than 100 years ago) is the least active day of the week for workers, and Christmas Day is the least active day of the year for workers. So to have Christmas Day coincide with Sunday makes it all fall neatly into place. On the other hand, Christmas Eve is generally a very busy day of the year for a lot of workers, including those who don’t usually have to work Sundays.
How many workers are forced to open up on a Sunday 24 December to get their business done before Christmas? Too many. You’ll be too widely-hated if you allow Christmas Eve to be forced onto a Sunday every year.