Why didn't Jack move to San Francisco or New York? It was the '60s, on the dawn of the Gay Rights movement, he should have figured out that not everywhere was Homophobic, especially not as homophobic as rural Wyoming or Texas.
Maybe because he had a stable home life (as stable as it could be, all things considered), affluence, and a family. He also seemed a country boy through-and-through, so moving to a place like San Francisco or New York might rankle with him.
It would have been much more difficult to see Ennis because of the distance, even if they only met up infrequently.
Because they don't identify themselves as gay, and would have seen the urban gay community as completely alien.
This. Just because you are bisexual or gay, doesn't mean you are going to automatically fit in a stereotypical gay community. It is possible to have your gender role and sexuality be the same, the opposite, or some wild combination. They aren't mutually inclusive.
Their professions did not really fit into urban life, and they themselves had probably no idea of how to survive and make a living in the big city. Also, in that time period, gay culture was not widely discussed in any event and by no means were the cities havens from anti-gay violence (still aren't in fact). This was why gay ghettos existed and gays tended to cluster together. The idea that they could have a "safe" life in a city might not have been something either of them would have really believed.