Because the world of programming is much, much older than both HTTP and REST.
The term [CRUD] was likely first popularized by James Martin in his 1983 book
"Managing the Data-base Environment".
Development of HTTP was initiated by Tim Berners-Lee at CERN in 1989.
The term representational state transfer [REST] was introduced and defined in
2000 by Roy Fielding in his doctoral dissertation at UC Irvine.
Besides, each letter in the acronym can map to a standard SQL statement but not so much HTTP methods:
The comparison of the database oriented CRUD operations to HTTP
methods has some flaws. Strictly speaking, both PUT and POST can
create resources; the key difference is that POST leaves it for the
server to decide at what URI to make the new resource available,
whilst PUT dictates what URI to use; URIs are of course a concept that
doesn't really line up with CRUD. The significant point about PUT is
that it will replace whatever resource the URI was previously
referring to with a brand new version, hence the PUT method being
listed for Update as well. PUT is a 'replace' operation, which one
could argue is not 'update'.
Finally the CRUD acronym's create, remove, update and delete operations can be implemented in many kinds of persistence layers, like object database, an XML database, flat text files, custom file formats, tape, or card, etc., which makes this acronym more universal.
Source: Wikipedia's article's on CRUD, REST and HTTP.