Earlier quoted context omitted.
I work on the core OS. This is the answer. Now it could be done to maintain backwards compat and have everything completely up to date, but the juice is not worth the squeeze and there are more important things to focus on (such as making the OS work/build in such a way that there won’t be inconsistencies in the future).
They tried modernizing with Windows RT, which was actually very smooth and functional if it wasn't so locked down.
So if you wanted an "unlocked" windows RT, couldn't you just have run windows 8?