More pay for easier work is one reason. You will make more money cast as a second banana in a reasonably successful sitcom than you'll make for a season of SNL.
Still, I'd consider it a labor of love. Basically nobody else does live sketch comedy anymore. I figured it has to be one of the hardest things there is to do. Plus you'd be a part of a long-standing franchise forever. Or maybe it's die-hards like us who keep the show's legend alive by word-of-mouth. I'd be like "Hey did you see so-and-so host SNL?" and they're all like "people still watch that show?"