Because in Unix/Linux like system it uses a "/" in paths but on windows it have to use "\" instead in paths and with some Windows system if configured properly to use "\\" and its damn hard to make the script auto detect if it Unix /Linux and Windows and it change like people change underwear like the the different browser have there own way to read a homepage but you have to convert all the files in my little forum to work with windows. But Unix / Linux is so powerfully if configured properly to read windows script but not good because if you have to have a secure script it is best to program it in what system it have run on.

And if it have to be auto detect the load time in a browser is slower because the scripts is much bigger and less secure. Too many things can go wrong.

the crackers will be happy because there are bigger chance for these it-criminals to find a back-door in the scripts to destroying you work.