Peak OSX / MacOS was Snow Leopard. From next version they hid scrollbars, swapped default scroll direction and started a steady move to make it seem everything should be hidden like iOS.
The success of phones and tablets appears to have made the desktop world afraid they were obsolete, so the trend since Snow Leopard and Windows 7 has been to make the desktop more like a phone or tablet. Windows 8 clearly went too far and provoked a revolt, so Microsoft improved things with Windows 10, but I think the Windows 7 UI is still better than Windows 10 in many ways.
I think that captures the why. Perhaps with a hope that a single UI between the two devices might be cheaper...
Windows 8 was a culture shock, but at least was clear in what it wanted to be. Personally I found it (and to a lesser extent the "mostly like a phone" UI of Win 10 and current MacOS) to be a mistake - I want a distinct and appropriate metaphor for a desktop rather than using it like a 27" tablet. People have dozens of apps and windows open, not one or two at a time.
Win 10, for me, is a mess. The whole flat thing went much, much too far, there's dozens of remnants of the Win 7 way of doing it in dialogues, settings, workflow etc. Win 7 is probably the nicest, and most coherent look Windows managed, but for workflow Win 2000 often had the edge! e.g. every version of Windows since 2000 has reduced the power of search, but made it increasingly obvious that it's there.
I am convinced if MS hadn't removed the start button but kept everything else as it was that Windows 8 would have been a success.
It was in the betas which I enjoyed using. But when it was removed it made the entire OS feel unusable to me. Whoever decided Windows Server 2012 should follow suit deserves just as much scorn.
