Allow tenants of Fannie Mae held properties to become owners without mortgage process if paying for more than one year

There are many tenants that have become renters paying Fannie Mae through no fault of their own - landlord defaulted on mortgage.
These are people who have been able to pay the rent which, in some cases, is more than a typical mortgage payment. They are those that have not been able to get a conventional mortgage due to credit, downpayment, not enough income to qualify or some other reason, but are capable of making their rental payments. Fannie Mae will take rent for a period of time and then announce house is for sale. Why can't the tenants become owners by Fannie Mae underwriting the loan and making the rental payment a mortgage payment. Proof they can pay is that they can pay the rent. Change the word "rent" to "mortgage".