Part of it is the customer base; you probably buy a Mac because Windows doesn't do what you want, and so the programs have to be different enough. One way to justify using a different machine is to have apps that put in more effort to be really good.
But also, it's a matter of copying what you see. When learning to program for the first time, a developer seems to do most of his or her learning on one platform, and copy that style for the most part (both the good and the bad, in everything from the design of GUIs to programming APIs). Regardless of what you think of either company, is there any doubt that Apple is better at coming up with good examples of design, than Microsoft? I consider Windows to be mediocrity breeding mediocrity; while this isn't 100% true, it's true enough for me to hate using the average PC.
Diversity is also a good indicator of quality; the more methods a programmer is familiar with, the more likely a program is to be well-designed. As a single platform, Mac OS X is pretty diverse; it comes with lots of tools, and is influenced by many different roots (Unix philosophy; NeXT/IBM with Objective-C/Smalltalk; classic Mac OS GUI elements; etc.).