I'll start by divvying Hippy culture up into two baskets: Fashion and Cultural Attitudes. The Fashion changed, because that's what fashions do. The attitudes, well some changed because the world around them changed, but some stay with us today.
Fashion is thing like clothing, art, music styles, hair styles. These are things that have a natural churn to them. Part of what makes fashion cool is its novelty. If something is not new and exciting, it really can't be fashionable. Pretty much by definition, anything your parents enjoy and/or are comfortable with is unfashionable.
So it makes perfect sense that Hippie fashions, whatever they ended up being, would be a reaction to 50's Eisenhower-era fashions. The best words there are "drab" and "conservative". We are talking very subdued colors, or pastels if you want to be racy. Short buzz-cuts for men, along with fairly well-fitted suits designed to not draw attention to the wearer. Long dresses and shortish hair on women. Popular music came from large band and orchestras.
So it makes perfect sense that their kids fashions would go against all of this. Bright colors. Long hair for everyone. Loose clothing for men, short skirts for women (or going the other way, clothing identical to men's). Music from small (5 or less member) bands.
And being fashion, it went the same way it came. By the 80's, simple long hair and loose unisex clothing became something your parents were into, not something cool and cutting edge. Music built around simple arrangements (particularly the simplest - folk music) became boring stuff kids would never buy.
Cultural Attitudes - This is a bit more complex. Hippies tended to believe in racial and sexual equality more than their parents, but it could be argued this is just a part of a larger ongoing movement that started in the postwar era and continues today. There were some things that came and went though, particularly their accepting attitudes towards sexual promiscuity and illegal drug use.
The sexual revolution I would argue was actually started by two developments: The birth control pill and legalized abortion. These gave women the ability to have sexual relationships nearly as risk-free as men. It also allowed them to compete on the job market and in cultural society at large, without the likelihood of being taken out of the action in the middle of things by an unexpected pregnancy.
The general perception with drugs among hippies was that if you don't get caught by the authorities, then no harm done. So if it feels good, do it. Why not?
What killed both of these attitudes in the 80's were the rise of really dangerous drugs and sexually transmitted diseases. Cocaine (and its little brother Crack) was the drug that turned "drugs" into scary things. Not only did they destroy a lot of lives (I know several cases personally), but they completely tore up the inner-cities.
For the diseases, we had Herpes and then AIDS. Herpes, had been around for centuries, but spread freely during the free love 70's (and got a nice hyping hand in the late 70's from a pharma company that had developed a treatment) It was pretty scary because its persistent and there's no cure for it. When AIDS came along it was far worse because not only did it not have a cure, but it didn't even have a treatment and it killed people. At first, a lot of people. This turned sex back into something you had to be careful about.