When writers write about young people, they're often motivated by their own Nostalgia Filter. TV writers tend to be busiest in their late 30s and early 40s, and (like everyone else) their tastes and preferences were formed in their teens and early 20s; by the time they reach the big time, what is modern to them is actually 20 years out of date. A desire to Write What You Know also plays an important part.
As such, shows that first ran in The '90s often reminisced The '70s, shows in The '80s carry a lot of cultural baggage from The '60s, shows that first ran in The '70s hark back to The '50s, and shows in The '50s and The '60s had its nostalgic setups between The Gay '90s and The Roaring '20s (probably because the '30s and '40s hadn't featured many things people wanted to be nostalgic about).In another ten years, this will likely happen with the Turn of the Millennium or continue the current nostalgia surge in things from The '90s (see the altered version of this trope in The '50s and The '60s below).
At the start of the 21st century, this can also be seen outside of nostalgia in how some works seem to suggest that they took place in The '80s and steadily into The '90s when they are supposed to be set in the present-day or a little earlier by having the "cool kids" still rap and skateboard and the lingo is still Totally Radical Jive Turkey (even in works from The '90s and The '80s set in what was then The Present Day, where it was not relevant to begin with).This is closely related to the fact that such franchises as Transformers, Teenage Mutant Ninja Turtles, and Masters of the Universe are getting revamped around twenty years after the peaks of their popularity; in fact, G.I. Joe: The Rise of Cobra (2009) was a revival of G.I. Joe: A Real American Hero from the 1980s, which was, in turn, a revamp of the original Joes from the 1960s.
Another reason for this trope is a long tradition of Executive Meddling. Until relatively recently, American entertainment media was heavily censored compared to that of most other cultures, first by The Hays Code (which wasn't relaxed until the mid-1950s at the earliest and not scrapped entirely until 1968) for movies and then by the Federal Communications Commission (whose standards weren't truly liberalized until the mid-1990s) for television. For comics, The Comics Code limped on until 2011. That is why, until about 1996 or so, American entertainment was often behind the times in portraying political, social, or cultural change; sometimes, especially when it came to topics like homosexuality, it wasn't even allowed.
A special caveat is needed when it comes to Japanese Media: due to the grueling work loads that Japanese creators face compared to Western media production (which in and of itself is no picnic), they often have little to no chance to interact with current trends or pop culture, and have only their own prior experiences to draw upon when it comes to writing about present-day youth—so in this case, writing two decades behind ends up being an Enforced Trope instead of the writer preference described above.
