The same reason radio formats evolved from the original Top 40 and MOR, Middle of the Road, formats and began targeting demographics and psychographics.
The old Ed Sullivan, something for everyone, was the model for a while with Glen Campbell, Andy Williams, Sonny and Cher and the Smother Brothers. Then the skit productions like Jackie Gleason, Red Skelton and Carol Burnett ruled for a time.
Ed Sullivan broke new ground with Elvis and The Beatles and that show on Sundays was my family gathering. It was not a Sunday without that and the Chinese acrobats and Topo Gigo, the Italian Mouse. And my Dad stating that those boys with this Moe Howard haircuts were going nowhere.
Stand up comedians weren't stand alone, they needed those programs as Richard Pryor, George Carlin, Robert Klein and Alan King got their starts there.
Late night TV replaced the old variety shows because variety had become passe. The Tonight Show took over and spawned Cavett, Lettermen, Colbert and others.
Interestingly, several years ago a brazen and desperate broadcaster tried to bring back the old Top 40 format, like your iPod on shuffle, and that failed because it is as much now about what we don't like as what we like.
I do wonder if they could bring that old formula back with someone as universally recognized as Tom Hanks but the expense would be huge as would the risk.
I think all of these Simon shows like AGT have replaced that variety format and are less likely time shifted.
