Having worked numerous Christmas Day shifts in a pre-railway job... it was great. Triple time, hardly any work, get away from the family.
As has been said above, there are cultural differences the world over. Us Brits take Christmas Day more seriously as a 'day of rest' than most other countries, and travel demand is low - look at the traffic on the roads as evidence. Most of the rest of Europe treats Christmas Day in a similar way we treat Easter, just without the bunnies.
Boxing Day is a different matter, and I think there is a good case for Boxing Day services on busy routes. However it is in very few franchise agreements (or staff agreements), so services will only run if they can be resourced and turn a profit. I know that at last one SE commuter TOC was offered unfettered access on a Boxing Day a few years ago as there were no engineering works on its route. It declined.