SO shouldn't be used as a gauge of popularity of a language or framework. Well established languages/frameworks will not see a lot of new questions being posted there since they have an established set of answers. Newer frameworks/languages will see a lot more activity there building up a base set of questions/answers.
The other angle that SO can misrepresent is that inexperienced developers are using the site more than experienced devs. If you know your language/framework/libraries pretty well, then you are mostly going other places for help like API docs or maybe directly to the Github repo for a library and asking committers since you are much deeper into the language.
Another reason could be that SO itself is trending downward. I know that there are people, like myself, who don't often use it or who just use it as part of a set of google results for the topic they are researching.
The other angle that SO can misrepresent is that inexperienced developers are using the site more than experienced devs. If you know your language/framework/libraries pretty well, then you are mostly going other places for help like API docs or maybe directly to the Github repo for a library and asking committers since you are much deeper into the language.