Because it's SO GOOD. And crazy. Mainly a little crazy. Or a lot crazy.
Listen to that banter. It's crazy. Sir Percival? Asking Bing Crosby if he likes contemporary music (answer: obviously not, it's Bing Crosby). Ohhh my God it's so amazingly weird.
But then it gets all awesome when they start singing. Well done, Bing and Bowie.