After WWII to sometime in the 90s, the personalities of the decades were heavily influenced by the age of the baby boomers -
50s - the boomers were children, and their parents were enjoying the post-war affluence
60s - boomers were in their teens with the idealism that is characteristic for that age.
70s - boomers were in their twenties, and starting to take responsibility for their life, taking jobs and finding life partners. The idealism of the late teens/early twenties age was still present.
80s - boomers were getting well settled into their life, and starting to have children of their own.
After the 80s, boomers lives weren’t changing significantly from their 30s, and the decades started to blur after that.
Note, not all people who were born in the baby boom went through the same experiences at the same time but the baby boom generation was such a huge bubble of people sharing the same limited age range and same rites of passage that if only half or even a quarter of those people have the same experience at the same time, it is like a rock dropped in the pop culture pond.
Technology was going through similar growing pains in the period that Stuntman Mike refers to. Movie technology (except for special effects) had mostly matured by the 70s, but the special effects were still maturing into the 90s and 2000s.
TV technology started in the 50s, and evolved through color TV, cable TV and onward.
Music technology developed starting with electrified instruments and production tools, and evolved until the synthesizers and drum kits of the 80s allowed people to sound like a full band with one or two actual performers. Grunge evolved as a rebellion against the heavily synthesized and produced music of the 80s.
Each change in technology was picked up by some artists, used to express their message, and other artists picked up and riffed on the new messages, images and sound.
Bottom line there is that a lot of the progression of pop media up until some time in the 90s/2000s was driven by changes in media technology, and when those changes flattened out, the changes stopped being tied trying the new technology, and those producing media were more about either picking up on whatever trendy or what they liked, because there is no longer a technology barrier to producing whatever you are interested in producing.
Major events also played a part in producing pop culture, starting even earlier:
WWI led to the jazz age of the 20s
The stock market crash led to the Great Depression
WWII led to the baby boom and the prosperity of the 50s.
JFK’s assassination played a role in the culture of the 60s, and the civil rights movement also played a huge part.
The oil embargo and shortages of the early 70s probably contributed to the Green movement.
I can’t think of any event from the 80s - unless it was the election of Ronald Reagan! - or the 90s of significant effect. The next major event was 9/11, and I can’t think of any major event after that.
Now, this is a very U.S.-centric maundering, but it should be remembered that the U.S. has been a big producer of pop culture since the 50s.
One more thing that has contributed to the lack of obvious pop-culture trends starting in the 90s is the advent and growth of the internet. This has allowed pop-culture to fragment, as people are able to find their tribe, their music and sometimes their preferred movies, TV, and other visual media, rather than only having what is physically distributed to the area they live in.
Finally, it’s hard to see the trends until you’re distanced from them. I remember watching videos and movies in the 80s where people seem perfectly normally dressed and coiffed, and now I watch those same videos and movies and go “OMG, look at that hairstyle”