"perhaps this Forum is just part of a wider trend"
We still get plenty of posts in the other forum areas, it's Speakers Corner which has seen a decrease in the number of posts. Lots of people watch the goings on without actually contributing - the so-called 'lurkers'.
Why has it happened? The world has changed, and nowadays people have other ways in which they can communicate. Mobile phones are the default communication devices of millions, and social media are being used by a generation of people who were still babies when our forums first came on the scene, early in 2000. Those people have never used web forums - Facebook first saw the light of day in 2004, followed two years later by Twitter - they went straight into social media.
Computer problems have not gone away however, and that's why the other forum areas remain relatively busy.