Everyone here is speculating about their content, but the simple answer is YouTube just changed how they count the view number. The change basically happened overnight, so it's not some slow attrition of views. They said in the WAN Show that while the view count halved, the number of likes hasn't changed (the view/like ratio doubled), and the revenue they earned hasn't changed (CPM doubled). All of this points that the same number of humans are watching, but what counts as a view in the "views" number just changed.