Revivals of a period tend to arrive around twenty years afterwards with reliable regularity. The interval is set by how long it takes a generation to reach the position of making and buying culture.
The people making the culture are the mechanism
Writers, directors, designers and executives typically reach decision-making positions in their thirties, roughly two decades after the period that formed their tastes as children and teenagers.
What they produce draws on that material, not as a strategy but because it is what they know deeply and what they associate with intensity of feeling.
The same interval applies to the audience with money, since people in their thirties have both disposable income and the strongest attachment to the period of their adolescence.
Adolescence marks the material more than any other period
Cultural attachment forms unusually strongly in the teenage years, and preferences established then tend to persist for decades afterwards.
That means a revival can address an audience with a shared reference set, and shared references are what allow a revival to be recognised rather than merely repeated.
Distance is what makes the period usable
Material from five years ago is stale rather than nostalgic, since it is still familiar enough to feel current and carries none of the pleasure of return.
Two decades is long enough that a period has become distinct, with a recognisable visual and musical identity that can be invoked in a few details.
It is also short enough that the audience remembers it directly, which distinguishes nostalgia from historical interest in something nobody present experienced.
Rights and availability follow the same clock
Catalogue material becomes cheaper to licence as it ages out of active exploitation, and revivals often begin with reissues and re-releases that cost little.
Successful reissues then justify new production, which is why a revival typically starts with old material returning before new work in the old style appears.
The cycle has been compressing
Continuous availability of archives has weakened the sense that a period is gone, and younger audiences encounter old material as current rather than as revival.
That has produced faster and overlapping cycles, with several decades being referenced simultaneously rather than one at a time.
What has not changed is the underlying driver, since the people making the work still draw on what formed them, and that interval is fixed by how careers develop.