There are many reasons but some of the main ones are: 1. Most forum users are repeat visitors and are generally blind to ads. 2. Forum users tend to be a bit more internet saavy and generally don't click on ads. 3. Ad placements on forums usually aren't in the most optimal places as not to disrupt the flow of the forum. 4. Even the forums have a niche, the nature of a forum, makes it fairly difficult to target specific high paying keywords without interupting the natural conversation that takes place on a forum.
With all the points above, Even if there's a conversion, due to enormous no. of pageviews compared to clicks, the revenue generated will be low. In other words, if your pageview is less and u get the same no. of clicks, chances are you, u'll earn more from them. Facing the issue myself
Well said. What I can suggest is to create a homepage for your forum and place ads there.. see how it goes (you can add some news, announcements, articles in there for some content).
I think the reason people think forum earnings are low, is because the average forum user might like at 20+ pages a session. So forum owners see a large number of page views, but low revenue. They should be looking at unique viewers, not page views, because their 20+ page views a person, is equivelant to most people's 2-3 page views per person. So they see 10x the pageviews, but the same revenue as someone with 10x less pageviews.
Another very important reason is that you have no control over the content of the page. So you end up confusing the adsense spider on which ads to serve. Ultimately the ads might be relevant to small part of your audience and not most.
Forums have never been high converters with adsense. Loads of reason, mainly due to the fact that placement is not often within the reading pane so blending is hard. The main objective is to get the ads right where the reading takes place.