APFS is my theory too. Given that Time Machine would need to be wholly re-written to support APFS, and the Apple strategy is to move everything to iCloud, this

APFS is my theory too. Given that Time Machine would need to be wholly re-written to support APFS, and the Apple strategy is to move everything to iCloud, this makes total sense. I also expect that APFS will make superior alternatives to Time Machine from third-parties possible, so IMO in the long term this is a net plus. It is probably unfortunate for Apple that this news got out before Time Machine 2: iCloud Bugaloo could be announced, though.