Either it's difficult enough that the developers didn't want to spend the time and money to make their code compatible with the most incompatible OS currently available
I don't mean stuff that isn't compatible with Windows, I mean actual Windows programs where the authors expect you to compile it yourself with a particular version of VC++, or even with command-line gcc and makefiles (VC++ is a free download, what Windows developer in their right mind uses gcc?)
Why don't they just post a binary? There's only one Windows 'distro' (yes, there's various versions but they're all compatible).
It's like they don't want people to use their software.