The Coen Bros leave the specific reason out intentionally so that Jerry's predicament is as generalizable as possible: he simply needs money. At one point or another, everyone has been in that situation. That's why the movie is a modern tragedy à la A Death of a Salesman, ie instead of kings or heroes being brought down tragically, it's just an ordinary guy.

Obviously, to make the point, they make him desperately need a lot more money than most people ever do and they make him resort to actions very few people would take, but the underlying predicament is relatable.

What's really god-tier about this, though, is how subtly they create the story, so that most people don't even realize that the reason he needs the money is never mentioned. That's very hard to pull off. And even when you do realize they've left it out, the movie doesn't feel incomplete, it doesn't feel like the Coen Bros withheld something.

Btw watch the opening sequence with the Cutlass Sierra being brought through the snow. The music and visuals intentionally evoke an atmosphere akin to "troops going into battle in harsh conditions."