Like all good parties, it stopped being cool sometime shortly after I showed up (2005) and has been in steady decline ever since.
The real "problem" was arguably the pipeline of new members: people inevitably lose interest/time/ability to be regular members of a lively forum, but the fading of Tolkien from the forefront of the Internet as the movies receded from view mixed with the demographic shift as interaction on the web went from fora to "social media" meant that there weren't new members coming in behind us.
Combine older users fading out and no one new coming in behind, and you get a sleepier forum. We can still be roused (whatever its other sins, RoP was good for that last year!), but the steady influx of posting in my youth seems to be behind us. Why, I remember when a netizen could return to the Active Topics after reading a post and continually find new posts read all night--yea, and into the morning!