A number of things contribute to this actually..
1) Advertisers typically don't like to put their ads in the forum areas, because they typically pay by the IMPRESSION or ad view, and in a forum a user can easily amount dozens of page views a day while seeing the same ad. If you look at Anand's main page, you'll notice much more variety in advertisers. And plus, when there's no advertising banners to go around, there's a DEFAULT ad, and in this case, it's provided by MUSHKIN.
2) The internet advertising business has declined by so much in the past few months, that it's getting harder and harder for a stie to sell their inventory. I have no idea how Anand is doing nowadays, but for example a site like MaximumPC, a year ago they'd EASILY sell at least 80% of their inventory on not just their own sites, but on their affiliated sites as well, which means millions and millions of page views being sold.
However that 80% is closer to about 20-30% nowadays, which just illustrates how even the biggest of sites can struggle in the dot com world. This isn't even to mention the smaller networks and sites out there.