A bunch of early Microsoft folks were also Unix users and preferred using / as a directory separator but IBM insisted on using / for command-line switches, forcing use of \ as the separator.

This was a late-breaking change and for a few versions of DOS you could set SWITCHAR=- in CONFIG.SYS to get the Unix behavior. When set "dir /s C:\whatever" becomes "dir -s c:/whatever".

You can literally blame IBM for this divergence. Without them pitching a fit about it DOS/Windows would have had the same switch syntax and path separators as Unix.