<Looking into my crystal ball and playing with my Quija Board> It is coming to me, give me a minute, I am seeing...
I wish I knew, but I think it is a matter of people wanting to shoot other things/methods than the Challenges Topics. They only have a certain amount of time to play/shoot, and they prefer using that time for other types of shoots. Some, as is proven by the 52 in 365, don't like to be challenged by having to shoot certain themes/topics. Last year when the 52 started, it was more or less set up for each to set up Topics/themes for the weeks and try to stick to the list. Some did just that and a few at that time just said they would not use a list, but would try to shoot something each week. This year it has more who have done the same thing, without a list per se. It is not a big deal as there aren't any set rules for the 52, except each person sets up their own guidelines/rules and then tries to stick to them for at least one shot per week for the year.
Having said all of that, my personal reason for doing any of the challenges is to push my comfort zone to try to improve. I look at it like a school assignment which gets me off of my behind and out to meet new "Challenges" in photography. Ha!