Another thing to consider is that it's a lot easier for a blog post to achieve a great search engine ranking than it is for a forum thread. One strong blog post can generate more search traffic than 10,000 forum threads.
If you think about it, Google is far more likely to send people to a well-written article than to a more chaotically structured forum thread. And most people who use Google would probably rather be referred to an article too.
The advantage of forums is that you can get a lot of long-tail hits from the sheer volume and variety of posts, but in my case I get way more search traffic from my 1000+ blog posts than from the 1M+ forum posts here.
Even my blog post on "how to cook brown rice" generates thousands of visits a month from search traffic alone. No forum thread has ever come close to topping it.
Suffice it to say that Google loves blogs a lot more than it loves forums. Forums mainly compete for the crumbs that blogs and other content forms haven't already captured.