It could just as easily be argued that the forward slash already had a perfectly valid use as a division operator and appropriating it for use as a directory separator was unacceptable.
In practice, using the escape operator as the separator causes a lot more problems. That's because you use the escape operator when specifying literal strings, whereas the forward slash as the division operator never conflicts with the forward slash as the path separator.
It's interesting that in its internal APIs, Windows is perfectly happy to accept either slash as the separator. It's only at the command line level that you have to use back-slash. That's because they chose the division operator as the command line option delimiter, instead of the subtract operator, which is what Unix uses.