Just my opinion, but I think the story sticks to Ennis because it's his story.
Yes, the topic of rural homophobia would affect both Jack and Ennis, but it was suggested that Jack was not as affected by it, hinting that Jack had other relationships or encounters with other men. Ennis, on the other hand was a M/M 'virgin', so to speak. His mind held on to the image of the tough old birds who had been murdered, and it stayed with him all his life.
Even though I think most viewers of the movie, and readers of the story accept Jack's murder with a crowbar, it was meant to be ambiguous, but Ennis believed it was the crowbar, and it shows how ingrained the rural homophobia was for him.
Telling the the story with the focus on Jack wouldn't have had the impact that it did with the focus on Ennis.