Mostly because many people do not like the Groups UI, so there were some discussions on possible alternatives...
We also wanted to wait for Google, to see if they were willing to provide an official new home. After discussions with them, we decided it was best to create a new Google Group.
It will mostly be maintained by the community (mostly the moderators of the G+ Community and the G Suite GDEs) rather than Googlers but they will help make it more visible (eg. via a link on the Apps Script documentation).
Also, on my side I think it's best to propose a new home near the end of G+. If we announced it before, some people might have preferred to stay on G+ as long as possible and it might be best to do a full switch and not have both communities running at the same time. Because of that, I think it could make sense to forbid people to post more content on the G+ community and "force" them to go here.