Because failing to pay your bills in a timely manner or declaring bankruptcy can both be indicators or poor planning or low personal levels of responsibility.
There are plenty of folks who have had a legitimate reason to declare bankruptcy, and there are often people who are in dire financial straights. However, GENERALLY SPEAKING, the abilty to pay your bills and avoid financial issues indicates responsiblity while the inverse is also true. Its another snapshot into the "whole person" picture. FWIW...you have to do the same thing to get security clearances in DoD work. The idea is that if you are strapped financially you are A) under more stress and B) more subject to coercion for financial gain.