I believe it is because the Milestone is on more than one carrier. Verizon and Motorola Worked together on the Droid's software and they streamlined the process because the Droid is only through Verizon, but the milestone is, for the most part, carrier independent. Even though it isn't to one single carrier I believe Motorola still works with carriers to make sure the phone updates don't brake anything. Also, Motorola have done their part with the Milestone by selling it, and they probably dont make any money with ongoing support, unlike verizon, who want to keep their contracted users happy.

Thats my opinion of it anyway. and i own a UK Milestone.