I don't mean to say that a low income person absolutely can't have good credit, but that it's likely to be correlated due to how the system works. For example,

I don't mean to say that a low income person absolutely can't have good credit, but that it's likely to be correlated due to how the system works. For example, a creditor will ask your income before extending credit to you. The more unused credit you have available to you, all else being equal, the better your credit score. So that's at least one reason to expect that they are correlated.