I guess it is related to forums in general, which have been declining for some time now and there are no signs that activity will pick up any time soon.
Yeah its a general trend among forums (at least the ones I am pat of) so bitcointalk is not exception here.
People have gradually lost interest in writing/reading, preferring to speak on podcasts or make videos.
I believe this is the reason for this decline. In addition to the fact that more and more information about Bitcoin and the like, there is less interest in going to forums to look for information.