It's difficult to have LTE on an unlocked device in the United States, since each carrier uses their own frequencies.
It's more difficult than you think, because frequencies aren't the real problem - protocols are.
LTE is a data-only protocol. No carrier that I know of has implemented Voice-over-LTE (VoLTE), though that is the obvious next step and carriers are looking into it soon. So in order to have a meaningful LTE device, you need to be able to support the voice/SMS/MMS protocol of the carrier you want to do business with. This would also be very helpful in places where LTE is unavailable, obviously, which for everyone but Verizon is most of the USA.
So putting LTE in a US GSM phone is not all that useful for two reasons:
1. AT&T's HSPA+ is already pretty fast, and my wife's non-LTE AT&T Galaxy S2 can hold its own against my Verizon HTC Thunderbolt even when I have LTE locked and loaded. There's a difference between HSPA+ and LTE, but non-HD-video-streaming day-to-day operations it's not noticeable. If you have an unlimited plan and like to stream HD video, LTE will benefit you. Other than that, not so much.
2. The LTE footprint offered by GSM carriers is practically nonexistent at this point.
For Verizon, LTE is a big hairy deal. The difference in speed between Verizon 3G and LTE is HUGE. For GSM carriers that already offer HSPA+21 or 42, it's a significant difference but even HSPA+21 is pretty darned fast. So it's not as monumental an improvement.