Because the C standard is notorious for saying things like the minimum size of an `int` is 16 bits. Changes in the size of data types would create issues when calling the Windows API or saving data to disk. Heck, the program is from the very early 1990's. C wasn't standardized until 1989 and compilers were notorious for having their own interpretation of C (what we would call extensions these days). Finally, developers often optimize large programs with machine dependent code. In the early 1990's, a full featured word processor would be considered a very large program.

Even today, C is not a terribly portable language when compared to it's modern peers. It is portable when compared to assembly languages and languages contemporary to its creation.