Maybe it's due to the fact that it feels poorly constructed. All the stories fit yet, but not well. They feel badly done and not expanded enough maybe had they been separate episodes we could have got a better end product.
So what killed it for me you ask. well. the end sequence, so the house wasn't real, Hamm's there for the police, Hamm gets blocked by everyone, the police torture the cookie. It was predictable. It didn't pack the punch the end of The Waldo Moment did, it wasn't as sadistic as White Bear. In fact in felt diluted and clean. Maybe the feature length was too much to keep up and that sucks.
I wanted to love you Black Mirror: White Christmas, I did but you killed it for me. Let's hope if it comes back it goes back to it's superior format this doesn't work.
(Christmas posts will be tagged with tis the season from now, so be warned)