DOS originally didn't even support directories but was using / for command line arguments. They didn't want to change the option character and break stuff so they went with \ as the directory separator.
DOS wasn't originally created by Microsoft. They bought the OS from computer shop in Seattle.
This is a great video about the launch of Windows 95, that covers the history of DOS at the beginning: https://youtu.be/0Ol8ZSE-PEk. Worth watching if you're interested in the history.