Actually Knock was built in the 80's, long after the pope had been and gone. And I'd imagine shrine visitors represent a very small percentage of pax numbers (the airport isn't actually that near to Knock shrine at all).
What Knock has that Derry does not is a population base in all directions from the airport, as it is well inland. It is also situated near the junction of two national primary routes. Part of the reason that Derry serves fewer routes is because half it's 'catchment area' is sea. If CODA was situated half way down the road to Omagh it would actually be far more successful than it currently is.