Reliability (ETOPS regulations), as well as the fact long hauls are usually fewer larger flights meaning they use larger aircraft.

Twin-engine aircraft have only one working engine if one fails, and there are rules regarding how far they can be from the nearest suitable airport in that case.

In practice that means that each type of twin-engine aircraft has a limit: Can only be that far from shore. For aircraft with more reliable engines the limit is higher, so they don't have to pick island-hopping routes when crossing oceans.