In order to be on the fha approved list, a complex has to file a lot of paperwork. It takes time and money. They have to show the types of loans on all the properties in the complex, show all their financials, show that their savings and investments will cover any big expenses that come up, show how many units are rented vs owner occupied and much more. Many complexes that meet the requirements dont bother to file the paperwork as a result.
If a buyer wants to buy in a complex that can meet the fha requirements but which hasnt filed, their mortgage lender can apply for a one time exemption for the complex