They sent postcards cause it was the cheapest easiest way to communicate.
You have to pay the phone bill, and have a phone, and on some the ranches Ennis was on, that could have been a hardship. And Jack's wife controlled the money---hence she read the bills.
Besides---they never said anything but dates and times for "fishing trips" on any of them--so they were pretty safe----and out in the open like truth is the most perfect disguise.
Nobody's wives bother about fishing and hunting trips between buddies---mostly they are just glad they don't have to go themselves. (voice of experience there)
It's interesting finding an old thread, and then learn something new. I never thought about the postcards, thinking it's just the way they did things in the old rural days, when ranchhands can't afford to pay dearly for a landline that they hardly ever use, and before they had cell phones.

But why did they not write longer letters? I think being the laconic cowboy, Ennis didn't need a piece of paper and an envelop. Even the postcard was too much territory to cover. His first reply back to Jack was "You bet." That's all needed said. I love the way he wrote the "u" in You. It was a definite smiley, like so glad to hear from you, and so happy to meet with you.
There is also the element of openness. Jack and Ennis definitely want to keep the content to seem innocent, and as you said, out in the open, truth is the best disguise...little do they know that the boys don't go up there to fish...

well, Alma knew, but she peeked.
The card said:" Are we still on for November? The fish are jumping" it flashed by real quick.
As for wives who don't bother about fishing and hunting trips between buddies, that's an interesting comment. I'll bet after the movie, every wife and girlfriend will wonder, what DO you boys do on boys night out? Well, here's a clue if the "fish are jumping".

Seriously though, I guess it's what a marriage is about, trust. Wives trust their husbands on these boys night out, and husbands in turn trust their wives on their girls night out. What's tragic in Brokeback Mountain is that the true love is forced into telling lies by the circumstances of the time.
I never thought much about that "fish are jumping" until reading it here. That's Ennis way of saying to Jack the same as what Jack said, "Sometimes I miss you so much, I can hardly stand it."

Them boys don't say much but they sure get their point across.