Why can't AOE2 for Mac have crossplay with Windows?

Does anyone know the technical reasons behind this, if there are? I'd like answers from people who actually know about it, not speculative answers please. If you don't know the exact reason for the game at least provide an example where code from mac and windows is not cross compatible, e.g. provide the C++ code and show it won't be cross compatible for an application between windows and mac?