A directive like #pragma once is not trivial to define in a fully portable way that has clear an unambiguous benefits. Some of the concepts for which it raises questions are not well defined on all systems that support C, and defining it in a simple way might provide no benefit over conventional include guards.
When the compile encounters #pragma once, how should it identify this file so that it doesn’t include its contents again?
The obvious answer is the unique location of the file on the system. This is fine if the system has unique locations for all files but many systems provide links (symlinks and hardlinks) that mean that a ‘file’ doesn’t have a unique location. Should the file be re-included just because it was found via a different name? Probably not.
But now there is a problem, how is it possible to define the behaviour of #pragma once in a way that has an exact meaning on all platforms – even those that don’t even have directories, let alone symlinks – and still get the desirable behaviour on systems that do have them?
You could say that a files identity is determined by its contents, so if an included file has a #pragma once and a file is included that has exactly the same contents, then the second and subsequent #includes shall have no effect.
This is easy to define and has well defined semantics. It also has good properties such that if a project is moved from a system that supports and uses filesystem links to one that doesn’t, it still behaves the same.
On the downside, every time an include file is encountered containing a #pragma once its contents must be checked against every other file using #pragma once that has already been included so far. This implies a performance hit similar to using #include guards in any case and adds a not insignificant burden to compiler writers. Obviously, the results of this could be cached, but the same is true for conventional include guards.
Conventional include guards force the programmer to choose a macro that is the unique identifier for an include file, but at least the behaviour is well-defined and simple to implement.
Given the potential pitfalls and costs, and the fact the conventional include guards do work, it is not surprising to me that the standards committee didn’t feel the need to standardize #pragma once.