'at no point during the EU referendum campaign was there an inkling that Northern Ireland would become the issue on which Britain’s withdrawal from the European Union would ultimately hinge.' Future historians will ask how entire UK political class failed to see this coming.
This isn't (just) a matter of condemnation. This was a colossal error with serious material consequences, & it was made by the political classes as a whole, with very few exceptions. And needless to say it was avoidable. How did it happen?
The question is not rhetorical. What explains the failure of most politicians, commentators and journalists to recognise what should have been obvious to anyone with even a passing knowledge of Northern Ireland and Anglo-Irish relations?
Part of the answer must lie in low public awareness/interest in GB. I seem to recall @stephenkb (who was a notably early exception to journalistic neglect of the issue) saying his articles on the border issue had the lowest readership of any he had ever published.
But it is the job of civil servants, professional politicians and serious commentators to look beyond what the public is consciously concerned about at any point in time. To identify problems before they become unavoidable. This just didn't happen.
More accurately, to the limited extent it did happen, voices were ignored. We can cite arrogance, complacency and ignorance, forces any government is subject to. But that just pushes the question down to another level: why was it on this central issue that they bore so heavily?
Just thinking out loud here, no answers. At the back of it all is a counterfactual I often think about: if HMG had realised in autumn 2016 how the border was bound to shape any future relationship with the EU, would it have done anything differently?