It was because of WSJ and Pewdiepie thing. Actually, some of the dickhead writers at WSJ took some things completely out of the context from Pewdipie's video (It was video about some Hitler joke) and portrayed him as anti-Semitic. Then those writer went on to making a WSJ post about how YouTube is showing ads against the racist videos. They also called out individual brands on tweeter that their ads appear against the racist videos. So huge brands started to pull out millions of dollar worth advertising from youtube. To put things back on track YouTube had to make the changes in guidelines and they are more strict now.

All this was done by WSJ because people are loosing trust in MSM and more people are trusting the news from the individual creators on YouTube. So they directly attacked YouTube. YouTube trying to save it's own brand so they are more strict with content and ad policies now.