Actually the \ character was chosen because the "/" character was used in DOS 1.0 for command line switches. And / was used in DOS 1.0 for command line switches because that's what they used in the DEC operating systems (VMS, DECSystem 10, DECSystem 20) from the 1970s. Remember that DOS 1.0 didn't support directories (all files were located at the root of the drive). They added directory support in DOS 2.0. Once / was used as a command line switch delimiter, it couldn't be used as a path separator, so they chose \ instead.
If you're going to blame the "/" character on anyone, it's not MSFT, it's DEC.
Oh and here's a little known fact: DOS (and Windows) allows the user to use either \ or / as a path separator.