I can't help but think the reason that the show seems so dire this year is because many of the most talented singers were put in the groups.

Some of the singers in The Risk and Little Mix could have been worthwhile contenders as solo acts. The blonde girl in Little Mix might have edged it into the girls' category, and there were definitely a few in The Risk who would have been better choices for the extremely weak boys' category.

maybe because the groups have been so rubbish in previous years that they actually wanted people who could sing in them and not 15 year olds who look good?

Perhaps, and it has produced two of the best groups in the show's history, but at the cost of talent in other categories that has probably damaged ratings.