#pragma once is quite commonly supported in many compilers nowadays, however, despite this wide support, it is still not a C++ standard.

Making #pragma once a C++ standard would allow the C++ community to compile their code faster, write less code to do the same as #ifndef and it reduces the chances of a name clash. The reduction of the chances of a name clash, is due to the fact that the programmer is no longer required to handle the macro of the file, as the compiler does this.