Because this pre-supposes that Sunningdale was in any way sustainable at that moment in History.
No? The point about Sunningdale is that the Provisional IRA retreated from an armed campaign to unite Ireland by force to surrendering its arms and accepting a power sharing agreement under the British state that had been brokered with moderate nationalists and unionists decades ago. It is a curious "stalemate" where one side surrenders all of its objectives and weapons to the other side that keeps all of theirs.
(And yeah, the same applies to the loyalist groups post-GFA. They're mostly fat old men who call themselves 'community activists' and complain on TV/to Arlene Foster and can't do much else.)