The Fianna Fail and Fine Gael parties represent the great majority of the Irish population, and they are ideologically the parties that agreed to the partition of Ireland - as opposed to the bomb-throwing Sinn Fein deadenders.
If the Protestants were to vote to join the Republic, Sinn Fein would lose all rhetorical justification for terrorism and 500,000 new Protestant voters in the Republic would obliterate Sinn Fein's representation in the Dail.