I think its pretty obvious. watchOS and tvOS stopped at beta 6 because they are not effected by these security holes. iOS was updated to beta 7 and then so was macOS. So I would assume macOS was effected as well.
Because there wasn't a 9.3.5 beta to test out the fix. Doing it with iOS 10 beta 7 allowed them to still test it before pushing it out 6 days later to the public.