WinRT can be used to create COM (DLL files) with C#/VB/C++. COM is pretty easy to do in C#/VB, so I am going to skip that story. WinRT with C++ is MUCH, MUCH, Simpler then making COM in C++.
So there is a reason: WinRT is easier to build then COM.
Now you are building a 30 Day To Launch Game and you find out you need to speed up a calculation. One of your friends writes in C++ and you need to connect with their work. WinRT is the answer.