IE11 didn't ship for Windows 8.0 it only shipped for Windows 7 and Windows 8.1.
Windows 8.1 is a free upgrade on Windows 8.0, more like a service pack release (when you look at the changes). Why not port it to Windows 8.0, it didn't make sense, 8.0 should fade out as people install the free upgrade that comes via Windows Updates (or the store, I forget which) and is a mandatory update. Opting out of mandatory updates means you opt out of associated upgrades.
This isn't a new or unique policy in Windows, look at iOS, you won't get WebGL on Mobile Safari without iOS 8, OSX doesn't back-port Safari and Andriod has a lot of versions with terrible browsers.