why do you believe Larry McMurtry entitled his book Lonesome Dove when the majority of the novel doesn't take place there?

There are two possibilities, here. One theory is that he just picked a name out of a hat and went with it. No deep meaning whatsoever.

The second and more plausible theory is that the title: Lonesome Dove suggests quite a few things. A dove is not meant to be lonely as they tend to mate for life, and the dove represents peace. None of this happens with the main characters.