1) nobody wants to read an "article" that has more ads than content when researching a topic (yes, ablock exists for us techies and I have it, but talking in the more broad common person sense).
2) nobody wants to read a forum thread with over 100 posts to try to find the one post with the real answer surrounded by a collection of bad answers and people saying "YUP, IT WORKS NOW" with zero context otherwise.
Make your content on-topic and valuable, and users will return. it is as simple as that. These are just two of the main things I see as devauluation of a lot of these web sites complaining about a drop in traffic. They're most certainly not the ONLY reasons, but it is two of the major points!