The move makes some sense in context. Computer sales were declining and it was looking (if you're Cueball) as if the traditional desktop/laptop computer was about to die out. The Surface was being developed, tablets were all the rage, PC gaming wasn't that big yet, and so Microsoft thought "alright, K/B+mouse is out, touchscreens are in, we need to update our interface to cater for this new demographic, but we'll still allow people to use a keyboard and mouse if they really want to I guess". That's when they went all in with Windows-On-ARM (remember Windows RT?) as well.

Obviously a completely missed shot in retrospect. What Microsoft miserably failed to understand is that smartphones were in, but touch screen interfaces are absolutely awful for power users. Ya can't really use Excel or write a book on a touchscreen. And ya don't need to pay for a Windows license to browse Facebook, Twitter, and passively consume some news and video. So the "middle segment" of tablets never really had broad appeal in general, and Surface tablets especially were the middle segment of a middle segment (with android/chromebooks on one side and the iPad on the other) so the Metro UI never had a chance to make sense for more than a handful of very lonely Surface users. No wonder they scrapped it a few years later when sales of touchscreen windows devices failed to materialize.