I actually used to work for Filemaker, mentioned upthread.
In addition to the risk of gray marketing, there is the not insubstantial cost of managing two separate products to release and market and support at the same time. I know they paid me a pretty penny and a half specifically because I could handle both Mac and Windows aspects of what I was working on, and keep them on schedule together.
Also note that there are, for many products including FM Pro, non-English versions released simultaneously as well.
In the case of FMPro, the source code base was pretty similar but not identical for Windows/Mac, and at the time anyway, the Mac users were mostly upgrades, not new users. The real growth was in the Windows market, but Filemaker being an Apple subsidiary, it wouldn’t do to be releasing Windows-only products.
Much of this is a management issue more than a software engineering issue, and not all companies have the management skill to see such a project through, and of those that do, they may decide it is not in their best interests to do so.