It's amazing that over the span of two decades, America went from this...

[Spider Jerusalem]

22. The two most overrated presidents of the 20th century, right there.

JFK didn't actually accomplish very much of anything at all. He was inspirational, that's about it; the day he died, every major bill on his legislative agenda was hopelessly bogged down in Congress; he more or less caused the Cuban Missile Crisis by going through with the Bay of Pigs and then not providing air support. He was probably more effective dead than alive, in fact; LBJ got the Civil Rights Act through Congress in part by framing it as JFK's legacy (but getting that bill passed was very much LBJ's achievement, and not Kennedy's). If you want a portrait of Presidential greatness and the potential of the office for good in the right hands, LBJ (Vietnam excepted) is a much much better example than Kennedy; Civil Rights Act, Voting Rights Act, Medicare/Medicaid, Head Start, etc.

And as to "what happened", the passage of the Civil Rights Act made Southern Democrats turn to the Republicans (see Nixon's Southern Strategy, best explained by Lee Atwater); as to what else happened? In 1970 US domestic oil production peaked. The USA ceased being able to produce enough oil to meet domestic demand. The Bretton Woods system that underpinned the postwar global economy with the US dollar as a global reserve currency, convertible to gold at a fixed rate, ended in 1974 in the midst of oil shocks and recessions. The rising cost of oil made Detroit, with their hulking gas-guzzlers, uncompetitive and American manufacturers started haemorraghing market share to Japan. The decline in industry of the '70's with the energy crises and increasing automation and resultant job losses led to what Jimmy Carter called "our national malaise".

The USA's unprecedented postwar prosperity was based on two things: one, the USA was the only major industrial nation left standing after WWII, and two, the USA in the immediate postwar era produced half of the world's oil. The recovery of European economies in the postwar era, and especially the economic rise of Germany and Japan, coupled with the decline in the USA's relative share of world oil production with the rise of OPEC, made the eventual decline of American industry pretty much inevitable. The immediate postwar economic boom of c. 1945-1970 or so was an unrepeatable historical fluke. One result of the recessions and stagflation and oil crises of the '70's? By 1980 a lot of people were ready to listen to a slick actor who told them that he had a plan for economic recovery (tax cuts, beloved of right-wingers everywhere). Like James Carville said, "it's the economy, stupid." (Only political leaders generally have a lot less control over said economy than most of them would like the voters to think.)