Google released several updates (such as "Panda") that greatly lowers the rank of domains that aren't linked to by major domains they deem to be trusted, such as those with edu or gov TLDs or whatever whitelist of domains they decided to add. There have also been updates that specifically lowers the rank of websites that are running forum or blog software. This was probably done to fight spammers but it has effectively killed off the long tail of topics.
This is admittedly speculation since the inner workings of Google search are not released to the public. But these findings have been corroborated by others on the internet.
I used to have a blog and several forums that used to rank highly for some niche keywords and over the years the search rankings started to drop off the front page while a single reddit comment with no content mentioning the same keywords would be #1.
I'd say that Google is very much to blame for this situation as they took the easy way out to fight spammers.