I think it's bit of an apples-and-oranges thing. Many of the current shows sort of stand on the shoulders of the earlier shows. I remember seeing the early SNL shows when they were first broadcast- at the time they were contemporary and cutting edge, very much like the Monty Python series of the same time. Now when I see the reruns they seem much more tame and dated, although still with some great moments. I think the loosening of what gets by the censors has helped too- the fact that broadcast TV has to compete with the uncensored cable networks has definitely had an influence there. The older shows paved the way for current ones- just think of some of those old Richard Pryor routines (on TV and in his movies) and the controversial nature of some of his material. We don't blink an eye at stuff like that anyomore.
The 70s shows were a reflection of the times, and times are very different now, so many of those shows haven't aged so well. I watch some of the old shows (lately things like St. Elsewhere and Hill Street Blues [80s shows, I know]) because they are period pieces and a reflection of an era to which I had a connection. I can see where people who weren't around then might have less interest.