The Rebels were in deep, deep trouble.  They had just destroyed the Death Star, but they had suffered a huge loss at the battle of Vas Vrogas, where Vader single handedly almost killed Luke and Leia.

The Rebels needed to rebuild.  And to do that, they needed supplies.

They sent Luke and Han Solo on a mission to get resources, but something went wrong.  Solo decided that he might be able to double the Rebellion’s money with a quick card game.