What a strange time to release something...a week before Christmas, after Black Friday/Cyber Monday, and after the "deadline" of December 15 by which most retailers guarantee delivery before Christmas.
It's targeted as a work machine, which isn't really subject to seasonal Thanksgiving/Xmas buying, not much at least. I'm guessing the demographic of people dropping $3-6K on shiny Xeon-shaped garbage cans as Christmas gifts is pretty small.
That said, I suspect they wanted to release this one earlier but hit some speed bumps.
Apple did say "December". They dident say you would GET it in December, but it would be available to PURCHASE in December... Same with iPhones these days... order online, 3-4 weeks before you get it...