Trailers routinely reveal plot turns that a film spends an hour building towards. The disclosure is deliberate and follows from what a trailer is measured on.
The trailer sells comprehension
Audiences decide whether to see a film in a few seconds, and that decision requires knowing what kind of film it is.
Ambiguity is expensive. A viewer who cannot categorise what they have seen does not become intrigued, they move on.
Everything in a trailer therefore works to establish genre, tone and stakes quickly, and plot detail is the fastest way to establish all three.
Trailers are tested like the film is
Cuts are shown to recruited audiences who are asked what the film is about, whether they intend to see it, and what they remember.
A version that preserves mystery scores poorly on the first question, and versions that explain more score better.
Because the testing measures intent to see rather than eventual satisfaction, it systematically rewards revealing more.
The people cutting it did not make the film
Trailers are generally produced by specialist agencies, often several competing on the same title, and the studio selects between their versions.
Those agencies are briefed on marketing objectives and are not responsible for the experience of watching the finished film.
Material is sometimes used that does not appear in the release, either from discarded scenes or altered in the edit, because the trailer is treated as a separate product.
The placement forces the structure
A trailer shown before another film reaches an audience who did not choose it and are not obliged to attend to it.
That environment rewards immediacy, and any structure requiring patience is wasted on a substantial part of the room.
Trailers preceded by shorter teasers exist to solve the same problem, capturing attention with a few seconds before asking for two minutes.
Where restraint is affordable
A film with a recognised property behind it does not need to establish what it is, so its marketing can withhold detail and rely on the title.
Directors with a following can market on a name alone, which is why some campaigns consist of little more than an image and a date.
Restraint is therefore a privilege of recognition, and a film without it has to explain itself, which is the point at which the surprises start being spent.