From what I understand, because there was very little travel to the western plains in those times, the pandemic had not spread to West Texas. At least not to pandemic levels.

The pandemic today has spread over the whole world simply because of mass travel (cars, trains, airplanes).

Back then there were no planes and few trains, with none in that area of West Texas and the only roads were dirt trails.

So, you could run from the virus back then. That is not possible today.