Reliability would be the driver here - ETOPS length is certified by aircraft type, and a whole host of tests and design work is done to show safety under that length of diversion trip after a "problem" that causes the aircraft to head for a landing site. If the aircraft is capable of it, it should be no less dangerous at 5 hours than 3. Also with AEDs on board, there is less of a need to be close to a hospital as well.