Also the exceptionally low population around Knock hinders that significantly.
Shannon's just down the road for most people (and others Dublin is just up the road) who use Knock airport, so the US and middle aren't too far away, compared to Inverness and many parts of England which are quite isolated. For example, Newcastle airport which serves the Tyne and Wear metro area, has a population of 1.12million, has no flights to the US (other than the odd charter).