Why does amount of debt or money owed have as much weight as credit history in determining a person’s credit score?

Because  debt to income ratio determines how likely you are able to make your payments.

AS your debt to income ratio become higher, the less likely the credit providers would loan their money to you since they would doubt that you have enough left from your income to pay them back.

As an industry standard, most credit providers in united states would see you as 'financially healthy' if your debt to income ratio does not exceed 36%.