Asking WHY sitcoms have changed is like asking why films have changed, or books, or TV shows, or fashion.
All of these things change over time, they adapt as society changes.
I think a lot of early sitcoms were based on "class" (1950s and 1960s), but as the class system has changed this is no longer funny.
Also, now we are very "PC" in this country a lot of humour from the 60s and 70s now seems embarrising.
Programs like Love Thy Neighbour (black family next door to white family) or On the Buses (two aging men chatting up young dolly birds) just makes you squirm.
"Mind your language", set in an evening class where people (black, asian, chineese) were trying to learn English, got most of its laughs by taking the micky out of peoples misunderstanding of English and got "easy laughs".
Some sitcoms (the good ones) still stand the test of time and Porridge, or Steptoe and Son, with excellent scripts, would still stand up today.
Both of them were like mini dramas with laughs and both Steptoe and Porridge had their serious moments.
Only Fools and Horses has its serious moments and was often more like a play with laughs than an out and out comedy.
But a comedy like say "My Family" is an old fashioned comedy that still works today, so in some sense they have not changed that much.