IBM licensed DOS from Microsoft. Microsoft bought DOS from Seattle Computer Products QDOS. That software got its command line switches using "/" from CP/M for compatibility reasons; originally, both CP/M and MS-DOS were available for the IBM PC.

CP/M borrowed the convention primarily from RT-11, the OS for the PDP-11, although it wasn't consistently followed there. Programs on RT-11 were responsible for parsing their own command line args, and not all of them used the same convention.

Inside Windows itself, most APIs accept either forward or backward slashes in paths (even both in the same path) without any special incantation. The problem is mainly at the application level where the whole forward/backward slash thing gets messed up because technically you should accept either one from user input and most app code expects one or the other.