It was because developers historically were familiar with Windows and would just default to making a Windows product. You want a POS interface? Your developer is probably going to hand you a .exe and not a .deb. Then your next move is to tell the hardware division to put that .exe into production systems, at which it is too late for the hardware division to argue you just chose the more expensive option without thinking.

This is changing, particularly as many platforms make it trivial to compile for different OSes.