thalmin
Retired game store owner
We were still providing DMG playtest feedback in mid-August.I get the impression that they've been working on the books one-at-a-time due to the small team size. So they probably had an outline of things they wanted to put in it, and some half-tested rules modules, but they haven't been able to dig their teeth into it until the MM went off to the printers.
I suspect they discovered they needed more time to make all the rules options jive together.