Probably a lot of factors but I'm going with "age".
My guess is the average age of forum users has increased and they aren't making as many things anymore and those that still frequent the forums come to see what's going on not to post their now absent new creations.
The younger people that would normally backfill are not computer forum oriented. They like looking at video things on Youtube and this is where they see how to do things. They also like doing things with a phone and IMO, doing much on a forum is a PITA with a phone but then again, I'm part of the age problem.
I can draw this age thing into a parallel with the small town I live in. In the old days, the town populace was young and there were lots of activities happening. Now that the town is noticeably older these activities have fallen away; the old people are unable or now don't feel like doing such things anymore.