Reasons to do it in C++ - mobile and console is already in C++, so you can unify ports. Can make fancier graphics - it's Voxel graphics right now, but the next Minecraft could look like Destiny while retaining all the sand-box nature of Minecraft.

You use scripting languages to save dev time - Microsoft can just throw more devs at it. They spent 2.5 billion on it - labour is cheap at that scale...