A good short story has a strong hook at the start to draw the reader in. It doesn't have room for extraneous details. And the pacing is crucial - it should move along quickly without feeling rushed.
Sorry, I don't have the specific date when 'The Hitchhicker' was written. But you could try looking it up in a library or on dedicated literature websites.
Maybe because the author wanted to convey a specific emotion or theme in a concise and focused way. Short stories often have the power to make a strong impact with fewer words.