The system call used to write to the console will vary from OS to OS, so right there is one example of why software is OS specific.

Operating systems provide entry points through system calls that allow user-level processes to request services from the kernel. It is important to distinguish between system calls and library ...

Though “barely an operating system,” DOS still matters (to some people) Ars talks with members of the FreeDOS project team with one thought in mind: "Why?" ...