Generally with any US phone updates are put together by the manufacturer (Motorola) and approved by the carrier (Verizon). There is some crossover- carriers wanting to put sponsored and their own apps on it and restrict certain features and manufacturers having internal quality control concerns- but in general once the OS version is released by Google, if and when you get it on your phone is between those two parties.
Motorola officially chooses which phones will get the updates, and when. For some carrier devices the carrier is more involved than others (CDMA carriers are more involved). Usually Motorola will make up some builds that they find suitable to be OTA'd, then give them to Verizon for their testing. They get feedback, fix issues, then reapply them and when its finally deemed ready to go, they roll out the OTA in waves.
As you can see this can be a long process. Made longer by the fact that companies like Motorola make sooo many phones. As we've seen this past week with Moto's release schedule for ICS, many devices are targeted for a Q3 update, which is an absolutely atrociously long wait.