That would have created a legitimacy problem for the EU. Brussels is hard set on pushing for "an ever closer union" and London has had a policy of ratifying new

That would have created a legitimacy problem for the EU. Brussels is hard set on pushing for "an ever closer union" and London has had a policy of ratifying new EU treaties without a referendum. So a majority vote for Brexit that fell short of a prescribed threshold would create huge problems for future EU treaties.