If there´s any reason why there is a delay, I would put my hand in that the dashboard team is waiting on the security team to look into the kernel a little bit better in terms of the firmware hacks and disc jitter rumored to be in this update.
Think that´s just a conspiracy theory?
Think again. Remember that useless update way back over a year ago on January 30th, 2006?
Here´s the feature list:
http://www.xbox.com/en-US/community/news/2006/0130-autoupdate.htm
Exactly .. nothing. The real reason for the update? To patch the kiosk disc from booting.
http://kotaku.com/gaming/xbox-360/xbox-live-update-fixes-kiosk-hack-151635.php
Not done yet. Also remember that unannounced, also-useless update on January 9th of this year? Nobody knew it was coming, nobody knew what it was, people had to ask Major Nelson AFTERWARDS what it was.
The real answer was they were patching the holes in their hypervisor in kernels 4532 and 4548. Major called it "performance and stability issues"
http://www.majornelson.com/archive/2007/01/09/today-s-dash-update.aspx
I´ll let you decide for yourself, but if history is repeating itself, the reason for the delay is to try and block backups from running.