I see a high majority of the reason being that all the information is out there already. Let's face it, the ones that are asking the questions 95% of the time are ones that are too lazy to search, don't know how or what to search for, or want it spoon fed. If they actually searched they would find what they are looking for without even having to ask.
Having all the information available already is slowing making forums obsolete as more and more people figure out they can get the answers without having to register and make a post. Once the rest of the them catch up, forums will end up dying out.
Look how many people come on, ask a question, then never post again once they fix their car. Most don't want or care to join a forum for the community atmosphere like the rest of us, they just want their question answered and move on once they get it.