Hey guys, here is my theory for today.
Verizon agreed to the exclusivity, because that is what Google was offering, and the tech folks in the company talked the execs into it. The execs looked at all the other Nexus phones, and determined it would be a quiet launch, and kind of an afterthought of the product line, so the fact they had no extra revenue potential and less control wasn't too big of an issue. (Plus from the rumors of tension, there were some unaddressed or misunderstood issues in the contract.) Originally they were going to launch it in early-mid november, whenever Google got it to them. And then it happened. The phone caught on like wildfire, and had MASSIVE ground-level support. Verizon execs realized it was not going to be an afterthought, it was going to overwhelm the rest of the product line. So they started pushing the launch back, back, back, until they got to a point when it was in a relatively dead zone for phone launches, and didn't overwhelm the rest of their phone line (that was more profitable). The launch is flexible (per "The Source") because they really are watching the RAZR and Rezound sales, and there is a point where they will be satisfied, or when they run into their self-imposed deadline, of the end of the year.