Well, they only made it in the first place so web developers could test their websites on Windows in a way that would verify compatibility with iOS and Mac. As Chrome (which is very similar to Safari, of course) and iOS came to be a very critical web deployment target, it was no longer needed. Safari for Windows wasn't built purely to have those users use Safari, nor was it discontinued to make people use Mac OS X: it existed so iOS and Mac OS X users would not be ignored by web developers running Windows.