I think it probably has to do with the difference in scale mostly, coupled with the comparatively sparse population that was mostly Mexicans, or just recently Mexican-Americans. The Anglo-American population was just starting to trickle into the West and I think that it ("the Wild West") was considered another sphere entirely from "America," and that people on the East Coast probably thought that anything causal began in the East and it's effects moved West. They probably didn't think that anything happening so far West was really intertwined in the war, especially when they are seeing entire towns quartering massive armies.

The difference in scale makes it seem less relevant historically too, I think (at least on the surface). Sibley's troops only numbered a few thousand, I believe, compared to General Lee commanding 40,000 men at Antietam alone. And Sibley's campaign was short lived, constantly moving, and almost entirely unsuccessful. It certainly wouldn't have made good Confederate propaganda.