There's many possible reasons. 

- One would be that the general attention span has declined. Classic internet forums had descriptive text-only thread titles not adapted to short attention spans unlike the click-bait titles often accompanied with media such as a picture or a video we have nowadays.
- Another reason is that the web has become more and more centralized. It looks like people now prefer having every forum grouped in a single website, instead of having multiple specialized forums.

A smaller site with a tight community could become prominent again by covering and providing solutions for a niche market that is not or less covered in a more centralized website.
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