DOS uses / because programs written for CP/M, and which were subsequently ported to MS-DOS, used forward slashes.

when PC/MS-DOS 2.0 was released, with support for directories, it supported both forward and backward slashes for directory separator because Microsoft programmers wanted to use forward slashes (bringing them over from Xenix, including adding virtual "DEV" directory with device files), but for compatibility and user friendliness the default was \ for directories and / for options