There is no way to fix it. That is the normal progression of a niche forum going mainstream. That's the way of the Internet. The average poster's contributions trend toward the Internet's greater average, which is terrible. I've seen it happen on a number of forums that I've participated in.
This.
You WERE in a niche forum with many like-minded people. Now, more and more people of diverse cultures, different intelligence and education levels, different age groups, different financial class groups, are all joining up. There's going to be a lot of "stupid questions", a lot of repeated answers, a lot of facepalming. Just deal with it and move on. It's the nature of an open-ended forum.
Forums are not meant to be places of quality content. They are meant to be places of discussion. Start a blog and read other people's blogs if you are looking for 100% quality content.
Sometimes, but definitely not the norm.