It's not just one mistake, tho, is it. It's the latest in a succession of incidents where he has tried to undermine the independent judgments of bodies that have found serious examples of misconduct: Priti Patel, Robert Jenrick, Owen Paterson most obviously.
Of course there are other things he has done which in the opinion of many is undermining this country's reputation in the world and in Europe, but presumably he actually believes he is doing the right thing there so he isn't going to resign over those.