The reason is because, as we knew previously, the LG Nexus 4 is really a modified version of the LG Optimus G. That phone does support LTE, and LG used the same Qualcomm Snapdragon chipset as in the Optimus G.
That means the following, according to LG:
This powerful chipset is only available with a combined processor and modem and cannot be implemented separately. The modem contains 4G LTE capabilities, but is only effective when combined with other essential hardware parts such as a signal amplifier and filter in order for it to work. It therefore cannot be upgraded to 4G LTE capability through software.In other words, it has the capabilities that have been shown thus far not because we wanted it to, but because we wanted the processor, and that included the LTE modem as part of the chipset. It's an accident (sort of).
With the inclusion of the Qualcomm Snapdragon chipset, Nexus 4 therefore offers the same amazing processing performance as LG Optimus G but for the 3G market.
It's sad, because we still wonder why Google would release its new developer phone without top-of-the-line 4G LTE capabilities. Nowadays, folks aren't even trying to advertise HSPA+ as 4G (except for T-Mobile), so the lack of LTE really is sad on what is Google's flagship phone and the first to carry Android 4.2.