Part of the reason was that WinRT was aimed at tablets and other low power devices, where unrestricted Win32 API use could drain the battery too quickly. They were trying to put the genie back in the bottle to have more control in this new ecosystem, like Apple enjoyed on iPad, etc. Much of the weird signing and app store evaluation was to make sure your app used only the blessed APIs and wouldn't, for example, stay awake in the background listening on an arbitrary port and drain the little tablet battery.

Painful and nonsensical from a desktop standpoint but also kind of impressive in a way.