A film is on a service in March and gone in April, with no announcement and no explanation. This is normal, it is rarely a decision about the film, and the mechanics behind it are worth knowing if you rely on catalogues staying put.
Licences are windows
Most streaming rights are licensed for a fixed term and a fixed territory. When the term ends the title leaves unless the licence is renewed. Renewal is a commercial negotiation between the service and the rights holder, and either side can decline. Nothing about the film has changed; an agreement expired.
Because terms are negotiated in batches, titles often arrive and depart in groups. A studio's output deal covering several years of releases will move dozens of films at once. What looks like a curatorial decision is usually a contract anniversary.
Territory is separate from the title
Rights are sold country by country, frequently to different distributors. A service can hold streaming rights in one market and not in the next, for the same film, at the same time. This is why catalogues differ between countries and why a title you watched abroad may be unavailable at home.
It also explains why a service cannot simply make a missing film available on request. In the market where it is missing, someone else holds the right.
Music is the common obstacle
Recorded music in a film is licensed separately from the film itself, and those licences have their own terms. A soundtrack cleared for theatrical release and home video may not have been cleared for streaming, or may have been cleared for a period that has now ended.
When re-clearing proves impossible or uneconomic, the usual outcomes are that the title becomes unavailable or that it appears with music replaced. Television series of the 1970s to 1990s, which often used chart music casually, are affected disproportionately.
Exclusivity and withdrawal
A rights holder may pull a title from general licensing to place it on its own service, or hold it back before a theatrical re-release or a restoration. Some withdrawals are permanent and deliberate: rights disputes, contested ownership after a company changes hands, or a decision not to circulate a work.
What follows from this
Streaming availability is a snapshot of a contract, not a property of the film. If access to something specific matters — for teaching, for research, for rewatching a favourite — a physical copy or a purchased digital copy is the only form of access that does not depend on a licence being renewed. Purchased digital copies are themselves subject to the storefront continuing to operate, which is a weaker guarantee than a disc on a shelf.