Why do some programs have their own criteria including things like debt ratios when the banks already have those in place?

As we saw before the foreclosure crisis, lending and underwriting requirements can become lax with time. Just because a lender approves a loan amount does not mean that the homebuyer can afford the associated payment. To protect against inconsistent or loose borrowing criteria, many programs maintain internal underwriting policies that often dictate more conservative underwriting for things like housing cost and/or debt-to-income ratios.