There isn't a market for an LTE variant with HSPA fallback. Like Bell and Rogers, AT&T is only beginning to roll out LTE and has an extremely limited footprint. By the time that network is significant enough to equate to major sales, it will be time for a new Nexus anyway. IF AT&T had wanted it, we'd have an LTE variant in Canada, but they're not going to make one just for Canada's minuscule LTE market. In the meantime, HSPA provides significantly better battery life in a thinner device, and it works for everyone.
They went with the OMAP because TI made the best bid for being the reference chipset. It's not all to do with power, product support and battery life are important too.
Remember that because these are developer reference phones, Google needs to use reliable, trustworthy hardware. Anytime you release the absolute bleeding edge latest and greatest you risk significant product delays due to unforeseen issues, and hardware problems nobody has encountered because nobody has any experience with the hardware.
While they don't want to be behind the curve either, you're not likely to see a Nexus that blows away everyone else because they don't want to take too much of a risk with such an important device.