Well, it's in part because it conflicted with a major family event for Christmas. The girls are seen to be preferring their rich friends and a Caribbean vacation over being with the royal families.
Perhaps (and I wouldn't be surprised), even the royals see hanging out with the family as a key and crucial part of being a representative (working) royal. Given their father and mother's errors in choosing friends/business associates, the skills that Beatrice and Eugenie need to develop might be best learned by some personal sacrifice and at their grandmother's knee. That may be the way Charles views it - and his view is likely more important than anyone else's on this matter (save the Queen's, and she probably has multiple reasons for deciding to scale down the number of working royals, just as Prince Charles will have).
I don't think it helps that the York girls are seen as hobnobbing with the very rich as a main public activity this time of year, no. But, if duty is part of being a representative royal, they should respond to that call.
Myself, I think they have no chance of getting back on the list of working royals (for several reasons - it simply has to be pared down somehow and it's traditional for children of a reigning monarch to get those roles - and in the future, that will be William and Harry...) Since they cannot be working royals, they may as well cultivate their own friendships, find their own way in the world.
OTOH, if they wanted to get back on the list, they would have likely been better off hanging out with the family that day.