I remember they got a little worse each year (in my opinion) and I didn't even make it through the last one, but why were they so bad?
I personally thought they were trying too hard and that at least with the normal episodes you could almost relate to the family, but during the Christmas episodes they were made to be too stupid to even be considered funny.
It's because they're beating a dead horse for profit. The episodes still get ratings because they're only once a year. The show died when Nana did. The Queen of Sheba was the last great Royle Family outing. It had heart and comedy and the characters were all still in character.
The New Sofa and the two subsequent spin offs after it have been weak comedies, lacking in heart and has the characters all behaving like cliche stereotypes. Jim taking a dump in front of the family is just a big jump the shark moment for the show. He was metaphorically taking a dump on the series in my opinion.