Apple and Google have decided to throw away any compatibility with computers when they developed their mobile OS. You can't run the same application on your iPhone or an Android phone and on your computer.
Microsoft has decided to go the other way, sacrifice usability but maintain the compatibility. You could technically take an application from your computer and run it on your tablet or phone. They own most of the desktop OS market, so why throw it all away and make a completely incompatible device? Let's make the desktop OS more mobile-like and the phone OS more desktop-like, and we will meet in the middle and end up with just one OS to rule them all.
And then they found out that this does not make much sense. You don't need the application that's optimized for a 24" screen with a mouse and a keyboard to run on your phone. You also don't need the application that's optimized for a 7" touch screen to run on your desktop. So they disabled the support for Win32 on Windows RT, never even supported Win32 on mobiles, and the Metro applications for Windows 8 never really took off.
They ended up with a less usable mobile OS that is theoretically able to run desktop apps but nobody cares, and a less usable Windows 8 that is theoretically able to run mobile apps but also nobody cares. To save their business, they had to step back with Windows 10 because people did not want to upgrade from Windows 7, and they seem to have completely abandoned the idea of Windows running on phones.
And Steve Ballmer is proud of his achievements. LOL.