From Dan Ariely’s Coursera course on Behavioral Economics:

Poverty increases opportunity cost to every decision. When you are poor, every decision you make is a tradeoff between what you decide to do, and what you could have done instead. If you do X, you can’t afford to do Y.

That makes sense, but every rational decision carries a cognitive load. Rationality tires our brains out, and when you have too many rational decisions to make, you cognitive load overflows, and you don’t have the mental energy to make a rational (System 2) decision. So you let the default (System 1) system make the decision instead, and bad things happen. You fall for immediate temptations instead of planning for the future.