You are right that he shouldn't be fired over one result.
But a 25% win ratio over a calendar year means that any manager at any club would be under threat and at most clubs would probably been fired already.
Robinson's league record is atrocious and that's not even taking into consideration the entertainment value, style of football and poor quality players he'd leave behind.
Out of 62 league matches, Robinson has 20 wins, not great in itself, but 10 of those wins came in this first 22 matches. Since then we have 10 wins in 40 matches and that is very poor.