Because it takes a full re-write and re-architecture
If you've been a Windows developers, your apps are probably built to do things a certain way, and the Metro/WinRT system won't allow it. Any time you write a file or read a file, you need to completely reengineer those sections. In many cases, you won't even be able to rewrite the application. For example, you won't see a Metro/WinRT registry editor or defrag tool.
At best, developers will need to relearn concepts that they've been using without change since they learned to program.
J.Ja