Because we need to identity directory markings. Its as simple as that, we could use *, but then you couldn't put that in a Linux file name. Whatever character is used to separate Directories, is going to have to be disallowed in file names, understanding that basic concept and why it can never be fixed, will take you a few hows of research in bassic operating system file manipulation.
That still leaves me a little ashamed on behalf of the industry. Why should any characters be disallowed?