People got older and race less often so that takes A LOT of the conversation away.
Not as many younger people got into, and/or stayed in the sport, especially at the intense level so many of us were involved before.
There are way more recreational choices now than a generation ago and children's sports have developed to require a far greater time (and money) commitment than in the past that limits other choices for a family.
Another big consideration is that much information about products is available on the web and most people have easy access to that info. Much of what we used to ask about is more quickly and easily answered online from the usual sources and more people have learned that they can find it easily enough on their own.
So, less use of the forums.