Actually there is a very good reason why you can't install that on Windows 8.It's because it's already installed.It comes by default in Windows 8 under the name "Windows Defender".
In Windows 8 and Windows RT, Windows Defender provides the same level of protection against malware as Microsoft Security Essentials. You can't use Microsoft Security Essentials with Windows 8 or Windows RT, but you don't need to—Windows Defender is already included and ready to go.
edit :pecon got to it before me