Because at the moment he is the saviour of the Labour Party, the guy who wanted to end austerity and "for the many not the few" etc etc.
If he had won... he'd actually have had to find a way of delivering Christmas presents to all the children in the world on a sleigh in one night. There would have been endless fights over forming a cabinet, many of the Manifesto promises would have had to be binned and it would all have ended in Nice try, mate, but you've been rumbled.
As it is he is the noble hero - a la Scott of the Antarctic/Dunkirk/Charge of the Light Brigade (add spun defeat of choice).