why cant i install windows 8 on a macbook air 13-inch, Late 2010?

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You can install Windows 8 on your computer because it can run Windows 8, but you can't install the Windows support software that makes Windows run properly, so probably things like the Wi-Fi card, the sound and the graphic card may not work without these drivers.

That's just an Apple restriction. Apple has decided to give Windows 8 support to the most recent Macs, leaving old Macs with Windows 7. Unfortunately, to run Windows 8 properly, the only option is to install it on a virtual machine. Apart from that, if you want to install Windows 7 with Boot Camp, you can do it