WHY I HADN'T SEEN IT
It's a combination of Jack Nicholson-phobia and 70's Movie-phobia. The 70's are well known to be a dead zone of American films, with a couple of strong years at the beginning and the end, but an arid wasteland in between (Grease absolutely is part of the wasteland, but at least it's highly entertaining, if atrociously retrograde).
But the Jack Nicholson-phobia was probably stronger. Having now seen Chinatown, I am completely opened up to seeing his other films, while before it was just lodged in my head that he's a dirty old man whose acting style chiefly consists of 'smug.' Ah preconceptions, how you laugh...
