If it was profitable for 2 airlines to operate from SNN to NY year round (I'm not including BAW1/3 for obvious reasons) you can bet someone would be doing it.
Airlines only have a finite number of aircraft. SNN needs to make a new route more profitable than the alternative (e.g. DUB) for airlines. That's a big ask when so many people in it's catchment are willing to fly from elsewhere.
There is no anti-Shannon conspiracy here, it's a free market now (thank God!). SNN needs to stand on it's own 2 feet and can't point the finger at airlines and cry foul when they only operate seasonally, it's up to SNN to make them want to increase services further.