The CDMA radios are setup to work with Verizon Band Classes. Sprint uses the same 1900Mhz CDMA for voice/data, and 850Mhz CDMA is supported as well on Nexus 5. So it's there. What's not supported is Verizon's core LTE network on Band 13, but their capacity LTE layer IS which is Band 4.
So Nexus 5 is capable of attaching to Verizon's network, but Verizon chose not to add IMEI/MEID range into their database, which means Nexus 5 gets rejected from their network the second you try to authenticate... Sad.
The problem isn't Verizon or Google, it's the US government. They don't make rules allowing you to choose the phone you want on the carrier you want like Europe does, whom require carriers to allow any phone or tablet that's compatible with their network to be registered on their network.
Right now the FCC has said Verizon has to allow compatible devices on the LTE 700MHz spectrum that they sold them, but only if the network deems the product "safe for their network". This means Verizon could make up any excuse (such as saying KitKat was not "safe enough") to prevent the device from certification on their network, so Google bent and changed KitKat to meet Verizon's specifications of "safe" even though no other carrier or law requires these proprietary standards for the spectrum. The standards are mostly just Verizon putting it into their bloated tracking system and hampering usability (such as easier throttling) or making it easier to track you for marketing. It's a joke and the US government is still clowning around.
Nope... That's not it...
The voice network doesn't use 700 spectrum. Therefore it doesn't need to comply with block c rules.
When VoLTE happens and Verizon switches to it then any LTE device will work provided they support the technology.
case in point...
Nexus 7 works just fine, but since the code for activation was messed up by Google(yes popular belief is Verizon did it, but it was Google and Google bug pages they admitted they goofed and the next update will fix it, and it was fixed). Your sim had to be activated before putting it into a N7 because the N7 couldn't activate the sims.
Does the N7 on VZW still have an open bootloader? Ive had a feeling the MAIN reason VZW won't let you bring an N5 to the "party" is due to the open nature of the phone. VZW seems to have adopted a strict locked down bootloader policy in recent years. If the VZW N7 still has an open bootloader then this smacks down my theory.
Ive been a VZW customer for 15 years and was about to switch carries due to this policy before I found out the bootloader on the early S4's was exploitable to allow me to use the device however I chose, so I grabbed one right when the OTA that patched the exploit started rolling out while I could still get one with the old software. I have a feeling when I am ready to move on from it in a couple of years there wont be a phone on VZW with such options and I will be forced to switch carriers. Which sucks because I have been a happy customer for so long. Take note VZW! You have customers who are more than happy with your service, but don't want to have the devices we pay a metric butt load for locked down. I know we are a minority and you probably don't care if you lose the few of us, but we ARE here.