I think Verizon wants to distinguish themselves from the other carriers, hence the droid branding for example. That may be only one reason why they don't get all the top phones all the time; Verizon might not be able to get all the same phones as everyone else because whether it's Nokia, Samsung, or HTC, the manufacturers may not be willing to make every phone with different radios, antennas, and the software that is required to run them for CDMA only. Most of the world uses GSM, so while Verizon is the biggest carrier in the USA, it may not always be worth it to manufacture for them in the grand scheme of things.
Then you also have the supposed control demands made by Verizon, but as I understand it, most of the other major US carriers are similar in the bloat and proprietary software the demand on the phones.
Who really knows what goes on in the minds of those who are in control?!?!?!