More or less there was a shift in how Youtube allocated ad revenue, where instead of paying per view on any video over 10 minutes, they pay based on watch time alone (which makes sense with how they added mid-roll ads instead of just pre-roll ads). This incentivized longer videos, or at least didn't penalize them, but then more recently there was another shift in their recommendation algorithm to push these videos harder (I suspect because they found it increased watch time and thus ad impressions overall), and that really pushed the long-form videos into the mainstream.