Probably because the ADF offers a lot of fertile ground for exam questions. They should have all been shut down years ago. I bet if you took up a collection from airline pilots to shut down the ADF/NDB approaches at rural airports you would probably get enough to to pay for GPS app at all airports - I'm speaking of course of pre-glass airline pilots. Nobody like shooting those NDB approaches in the sim. To say the NDB is a non-precision approach is an understatement - it's an emergency proceedure