Google likes High Quality contents and they gives more preference to it as per their policy but forums having not more high quality content but they mostly ranked high.Can you discuss it why?
In my view Google like forums more than blogs i.e. because there will be lots of discussions by number of people. Different types of answers from the people of different places and ages, may be this is just the reason google like the contents of the forums.
I think Google treat them fair and they have difference in indexing and algorithm to index further these sites. Forum could gain reputation and authority faster than blogs.
Forum normally treated as discussion board. Google love to created community and they put those time of community on the top search where group of people love to share their views.
Forums are ranked much higher because more people discuss there. Therefore, keyword density in a forum is more than in blog(not always) whereas or a blog, only one way communication traffic is there and Google ranks it lower.
It not only consider the quality content but popularity also matters. Forums rank well because their popularity is much better than Blogs.
Forums appear high on the search results because they frequently get updated with new content, you know lot a people continuously participate in these discussion boards which is why they tend to get rank easily, in comparison to a blog.