4 Answers

They have gotten into the habit of doing whatever suits their purpose, right or wrong, to the point where it no longer matters to them. Their conscience has become dull.

Doing "what is right" often involves putting someone else's well-being in front of your own, or sacrificing some time, effort, or resources.  Being a selfish lot by nature (survival of the fittest, you snooze - you lose mentality), doing what is "right" can go against the basic instinct we have of maintaining our dominance over others. 
It is also a sign of weakness or fear when, for example, a person is "dared" to enter a store and steal something, or to bully another person.  
Doing what is "right" is often subjective, too.  What is "right" for YOU may be so wrong for someone else. 

human nature

One word . . . Survival.   I have observed that people want to be the best that they can be, but survival is the first law of nature.  And if one has a problem keeping everything together, one resorts to any and everything to survive.  Not pretty many times, but that what humans do.  Its hard to take the high ground when your kids don't have food, or you are sleeping outside of a 99 cents store, or you have no gas for your car and/or you are witnessing all your teeth coming out because you have not dental insurance.  I see this everyday in my city.  People will run over you with a car if it means that will survive a little longer.