Seems like Google is trying not to lose Samsung: https://9to5google.com/2016/06/13/report-claims-that-samsung...
Google is doing what they can to clean up the code base and separate concerns. Clearly many vendors have a technical problem merging their own changes with updated versions, and we've seen time and time again that vendors make promises -- they have the will -- that they don't live up to because they don't have the means.