Why does Windows use back slash and everything else uses forward slash?

TLDR: The assholes aren't MS, but IBM.

Because the first version of MS-DOS didn't support directories. Most of the tools for DOS 1.0 were written by IBM, and these fuckers used / for the command switches. Now when DOS 2.0 added directories, IBM wanted to maintain compatibility with their existing tools, so / for directories was out of question, and MS had to choose something else. They went for \.

Afterwards, they couldn't change that because of backwards compatibility. Customers hate it when their tools suddenly break. That's why Windows to this day is using \ although it doesn't contain DOS anymore.

Oh, and MS-DOS 1.0 didn't have directories because business. If a small startup like MS back then wants to sell shit to a megacorp like Big Blue, then quick delivery of an MVP is business critical. If that takes too long, the big corp will lose trust and ditch the startup, back then as well as today.

Given that DOS ran from floppy because harddisks were not affordable, directories were dispensable for an MVP 1.0 version.

@Fast-Nop TIL

It's a piece of mine as well. I like the fact that languages like Java convert / to \ when referencing files so of course I always use /. I even went through the hassle of using Cygwin just so that I can replace the default command prompt with one that understands forward slashes.