Well...
Our server software only runs on Windows because even though we designed the bulk of it to be platform-less-dependent, we ended up using some existing libraries in the company for database connectivity which rely on ODBC drivers and were designed with Windows specific code. The server could probably be ported to Unix/Linux, but we haven't had the motivation to try.
Our client software was built on MFC, so unless we do a full rewrite that's not going to move. (And we did start a rewrite at one point in Java but that got scrapped because it was taking too long and wasn't able to handle the size of the datasets we need to handle.)