I'm iritated too, but bear in mind: 1. It usually is updated at the same time as Android and iOS, so perhaps there's a legitimate reason, such as a last minute bug. 2. Imagine if MS prioritized Windows Mobile over Android and iOS and the critism they would receive then.

In this case my guess is while they usually wait for all the platforms to be ready at the same time, in this case there wasn't time to update Windows Mobile and as it was Christmas and 98% of their customers are using other platforms they made that calculation that they couldn't justify holding back the update while most of their customers had time off from work to enjoy the new release.

There is also the issue of sales they might get over Christmas with many people getting new phones, no exec in any company could justify to their boss passing on all that money.

All that said it is now 15.01.2017 and we're still waiting!

TLDR: This time unusually the release was too time sensitive to include Windows 10 Mobile before release.