From FindingBrokeback.com:
What happened to the Signal Gas Station scene? The most compelling reason for omitting it is that it postpones some of the film’s most artful imagery. The captivating encounter between Jack and Ennis outside Joe Aguirre’s trailer is one of the film’s jewels and, as such, deserved prominent placement. Additionally, the film, like the story upon which it is based, is masterfully tight. By removing this scene, Lee gave Brokeback Mountain added directness and impact.