To be somewhat less cynical, VSS and TFVC were not intended to scale to the size of Windows's codebase, thus they weren't used. And instead of inventing our own thing this time around, we went with (and make contributions back to) the de facto standard.

When I was in Xbox, we had a lot of things in TFVC: all the services code, most of the console and Windows apps, and many of the tools. Only the Windows-related bits were in SD.

If they didn't use MFC, WinForms and WPF what exactly were they using? Flash?

Win32

And WTL, Windows Template Library.

https://en.wikipedia.org/wiki/Windows_Template_Library

WTL is way old.

Yes, but it's far newer than Win32 and MFC. There's also ATL, Active Template Library.

UWP everywhere now though.

Is that true? Which larger app is written in UWP? Something like Office, Skype or Visual Studio.

Windows itself is gradually rewriting its shell components in UWP XAML.

The preferred Windows client for Skype has been UWP for a while now (though up until recently it was still often referred to as "Skype Preview").

There is a watered-down version of Office in UWP. The built-in OneNote is actually this UWP. It uses a different stylus filtering algorithm from the desktop version, which is why I don't use it myself, but rest assured it works just fine.