Hi and welcome jedibarrister.

For me it is obviously the choice of the scenarists and Ang Lee, in many places the short story is very graphic about (homosexual) sex and and that would have made the movie barely releasable in many markets.
There was already a lot of controversy about what is shown in the movie isn't?
BTW, Ang Lee experienced some distribution issues with is next movie after Brokeback, there were very explicit sex scenes in Lust Caution (IMO the actors were not faking). Ang Lee explained they were among the most difficult scenes he ever shot and we're talking about heterosexual sex here.
Let's go back to the topic,
I love the short story a lot.
I love the movie too if not more, because it was better then expected, I mean for me.
Adapting a book into a screenplay is always difficult and there are not much movies who are better than the books they were based on.
One example of a not so good movie, also adapted from an Annie Proulx story, The Shipping News.
IMO, they did very well with BBM and Annie Proulx once said Heath Ledger understood better than her the character she created.
I could hardly compare books and movies made from them, one example: I loved reading To Kill a Mockingbird, despite the many cuts from the original book, I love watching the movie too. Like Heath Ledger's Ennis Del Mar, Gregory Peck's Atticus Finch is a characterization that made movie history, this is what matters to me.
My two cents.