It's not just about the OP vs housekeeping.

Hotel guest getting off over housekeeping working for them?
The Hotel getting off over giving housekeeping a day off (paid or unpaid, even worse) while the guest is still paying normal rate for normal services?

If I employed household staff, I would give them a day off.
What if I had guests coming over? I would apologise for not providing them with a level of hospitality that I would have had it not been a holiday.

What if I charged my guests, normal rates, i.e. I ran a guest house and provided them reduced service?
What about giving Jewish staff a day off on their holiday, as well as Muslim Sikh, Buddhist and others a day off on their holiday?

Could the hotel staff have asked people if they wanted to work on Christmas day? Some might have been willing to work for extra pay.

More and more businesses open on Holidays now as there are people for whom Christmas might not be a holiday or are willing tow work for extra pay.

Most likely, it's corporate cost cutting measure than anything else.