Bem, antes da Apple mudar para a Intel, esse era o motivo.
Agora que eles usam a Intel - bem, eu acho que o nosso Journeyman Geek tem a resposta:
Oddly enough? Apple systems check for a specific chip and refuse to run or install without it. This is called the system management controller, and in effect is a glorified fan controller amongst other things. Practically speaking, this is the reason, outside of some other specific things that might be different – such as video card firmware for video cards and OS X specific drivers for various things (sound cards come to mind) that you can’t ‘just’ boot a vanilla copy of OS X right on your beige box pc. Of course, this isn’t that hard to get around, which is why your average OS X hosted VM host can run OS X VMs, and there are Hackintosh distros floating around.
Most Hackintosh install methods these days use variations of boot132, a bootloader that was provided when Apple was transitioning from PPC to Intel with some modifications. The original bootloader was open source, and built with some changes for Darwin. As an aside, there have been some attempts to repackage Darwin as an open source OS.
Apple supports a limited range of hardware you know will work. Otherwise, you’re going to have to scrounge up tested hardware or hack hardware into working. This is what makes running OS X on commodity hardware difficult. The SMC is relatively trivial to get around. Getting your unsupported sound chip (nothing like having your mic stuck at maximum volume on a laptop to ruin your day), video adaptor, and other hardware is the tricky part. If you have an AMD processor, for example, the stock kernel will take one look at it and panic like a mouse ran up its pants. In many cases, the solution ends up being building a new kernel, with patches off Darwin source (which is FOSS) and using that.
In short, the big problem isn’t the magic chip, it is OS X needing to play nice with the entire system.
Fonte: Por que é ainda é tão difícil instalar o OS X em PCs?