"There ain't no rest for the wicked" because the wicked seeks those things which can be found on earth and are willing to lie, cheat, and steal for sustenance. The righteous would rather die because they realize that there is endless joy in heaven and they would do nothing to jeopardize their afterlife. The "wicked" are the "hustlers" of life who do whatever they have to do to get what they want without regard for others.