I think it's interesting to note the differences between old SNL and current SNL.
Original SNL was ALL about parodying politics and current issues. Its hosts were mega-star actors, political figures and sports heroes. They pushed the envelope and pushed buttons. Weekend Update was political satire at its best and they truly treated it as a "news" segment. Their musical guests were bands who were right on the cusp of breaking out. And if they had already broken out, they were huge.
Current SNL seems to do only a little bit of politics (save for last year's elections), but most of the parodies are about pop culture, including Weekend Update (which seems to last for-ev-er). They still have a few big stars who host, but hosting has lost its luster - especially when you see them trouncing out Megan Fox and Shia LeBeouf (twice!?). Seems they're just nabbing a host who has a movie or TV show coming out. Their musical guests are only there to draw a crowd and do their latest single, rather than expose a new band or singer that's on the verge of breaking out.
I suppose in the end, though, that SNL is doing what it's always done - appeal to the next generation of younger 20-year-old males who didn't grow up on the "old" stuff and the "old" casts.
For the older folk, it might be that when they were young, watching SNL was one of those things they weren't allowed to do because they were supposed to be in bed. Watching SNL against mommy and daddy's rules made it even funnier - to see and watch "taboo", adult humor. Now that they're grown up and it isn't "cool" anymore to sneak downstairs and watch television, it's just not as fun and/or funny.