Are you stupid?
Because the short runway restricts the aircraft type that can be used for long haul to operate without weight restrictions which affects profitability.
United Airlines fly out of Belfast 90 minutes up the road from Dublin!
Cork is 3 hours from Dublin airport. Business traffic will not tolerate a 3 hour journey to Dublin and consequently fly via Heathrow.
Thankfully, Norwegian will be flying to both to New York and Boston on the 737MAX next year.
Enough said.