For Google, Nexus is a flagship brand that represents the best of Android, with Google leading by example to show other hardware manufacturers what Android can be. And though partners build the hardware, Google wants direct control of the software on Nexus devices with no carrier intervention. That alone means Google can't sell an LTE device, as there's simply no access to LTE networks without working with carriers in one way or another: Verizon and Sprint's LTE networks still require compatibility with their 3G CDMA systems, and there's essentially no such thing as an unlocked CDMA device. AT&T's fledgling LTE network runs on different frequencies than other LTE networks around the world, so Google would have to build a custom phone for just 77 markets in the US. Doing that without AT&T's financial assistance makes little sense.
Direct control over over the software released on hardware without carrier intervention? Hmmmmm... I wonder who has that? Oh yeah, Apple!
Whole thing's an excuse because they didn't make the same sort of deals Apple did with the carriers. Now it's fucking them over. Some great "open" handset ecosystem they got going there. The Nexus is the only true Android phone being sold in the USA and now it's also being crippled due to wireless carrier control. Keep enjoying your garbage skins and preloaded carrier bloat-ware apps that you have to hack off your phones guys.
People often fail to see the simple ways in which Apple's products have a level of freedom from the carriers no other handsets can match.