Because they're now "normal", so why would they continue paying for it, taking unpleasant injections, and enduring the side effects?
In this sense it's like any diet: they "work", but if you don't permanently modify your food intake, the weight comes back as soon as you go off the diet.
Another way of putting it is that people achieve their goals and wind down the usage of the drug that got them there.
I think that in a few more years the number may stay at 25% (or whatever) but that the makeup of the 25% may be different. That is, people will go off it and back on it if they see their progress reverse but that will happen to different people at different times.
In this sense it's like any diet: they "work", but if you don't permanently modify your food intake, the weight comes back as soon as you go off the diet.