Because Verizon would have to agree to it, sanction the phone, get their cut of the profits, and allow the devices to be activated on their CDMA network (which means they'll have to put each of those phones in the system). This works with Apple because Apple is in a way just serving as an authorized retailer for Verizon. The phones Apple sells for use on Verizon are phones that have already been purchased by Verizon. Verizon already has the phones in their system, Verizon has already approved the phones for use on their network, Verizon has already their proprietary software on there (there's next to no bloat on the iPhone, but there is other programming done in order for the phone to work on Verizon's network), and Verizon is getting their cut of the profits (which isn't much from the initial sale, but it ultimately works in their favor). With the model Google presumably wants to use (based on the Nexus One and the Galaxy Nexus), Verizon would be completely cut out from the process. If that's the case, Verizon isn't likely to put the required information into their system in order for the phones to operate on their CDMA network.
For strictly LTE use, this shouldn't be a problem as non-Verizon approved phones are supposed to work just fine as long as they have the required bands. But since there isn't LTE everywhere, a user would be limited in where they can use their phone.
So, what Google supposedly wants to do is very different from what Apple does. Google's plan would work just fine with GSM carriers, but not so well with CDMA carriers. It's part of the reason why a lot of people say the Verizon Galaxy Nexus isn't truly a Google Nexus device. Google can't just update the device as they see fit. Verizon still has to approve everything that happens to it. I doubt Verizon would be willing to relinquish that control.
