Why were the 60s so different from the 50s?

Vietnam.

Rock actually started in the 1940's Here's the earliest example I have found so far:

https://www.youtube.com/watch?v=WVHypczfLv0 1946

Here's one that's pretty wild in the 1950's:

https://www.youtube.com/watch?v=lYchr1yvgfk

And a Psychedelic classic from the peak of the Hippie era:

https://www.youtube.com/watch?v=hr3EVsdY6Yg

The premise that the 20s thru 50s were the same is flawed.

The depression in the 30s was a huge factor, prosperity was not common.

The war in the 40s resulted in full employment and shortages (due to rationing)

In the 50s the US was the major (only) country whose industrial capasity hadn't been destroyed in the war. The US was the worlds first super power. Because of this prosperity, the 50s were the first time that teens had enough money from their parents to become an economic factor. Before the 50s nobody directed advertising to the young people, because young folks didn't have any money.

The transformation of the US from an agricultural society in the 20s to an industrial country in the 50s lead to the changes that emerged in the 60s.

a large number of young people with the free time and money to be their own people.

Sorry mr, but Rock music was born in the 50s. Any educated historian on Music should know this.

Elvis Presley, Buddy Holly, The Crickets, Link Wray, Duane Eddy, The Everly Brothers and many others say Hi.

Literally not anyone, from The Beatles all the way to today's Justin Bieber would have happened without the mid to late 1950s.

It's because the country was purely conservative. Conservatives hate change.

They are ten more.