Because Verizon really, really wanted the iPhone so they were willing to give in to some of Apple's demands in order to get it. There was not nearly as much demand for the Gnex as there was for the iPhone, nor was Google as stubborn as Apple when it came to those demands (and Verizon probably wouldn't have caved on it anyway... they did agree not to mess with the unlockable bootloader, however).. so they got to push some more of their own policies on to this phone.
Also, it's been reported here previously (I'm not sure how accurate it is, so I'll preface by saying that) that Apple actually brings in on-site representatives from the big networks (Att, Vzw, etc) dedicated to test the iPhone upgrades on the networks on location while they are working on them, so the testing is complete, or close enough to it for VZ to give in, when Apple is ready to release the updates. Google doesn't do that.