/ vs. \
This is fairly well documented. MS-DOS 2.0 added a bunch of Unix-like features, most importantly directories. However / was already in use to start command-line options. Which was, by the way, compatible with CP/M and various IBM OSes, so it is not as if Microsoft invented use of / for options.
So ultimately it was decided to use \ rather than Unix / as a directory separator. HOWEVER DOS and Windows have always supported / as an alternative directory separator in the API, so usually / works equally well in a context where a command-line option cannot appear (i.e. open file dialog).
(For the true pedantic: using / instead of \ will NOT work with the \\?\ path prefix.)
Biting the hand that feeds IT
