Is there a reason an older Macbook won't run Mountain Lion?
Apparently it has to do with 32-bit graphics cards drivers, as they do not support 64-bit EFI. 

While Mountain Lion is compatible with any Mac capable of running a 64-bit kernel, the kernel does not support loading 32-bit kernel extensions. So, since the early 64-bit Macs use 32-bit drivers, Mountain Lion won't load them.

You can find out if your computer has a 32-bit or 64-bit EFI by running the command: ioreg -l -p IODeviceTree | grep firmware-abi

It will return either &lt;"EFI64"> or &lt;"EFI32">.