Why isn't the An-72 / Boeing YC-14 configuration used for airliners?
The extra lift provided by the blown flaps and the better protection against FOD is simply not needed in regular airline service, and the higher engine location will make maintenance harder and more expensive. Airport runways are long enough for most purposes already, and only tactical transports could possibly benefit from the enhanced lift and better FOD protection.

At least the An-72 and -74 is operated by a number of civilian and military entities. Note that all of them serve small, local airfields. Note also that passenger service with the An-72 has been discontinued in Finland since this type was not cleared there for passenger services.