Why does Android require you to revert back to factory version in order to disable a system app?
No, no, no.
Reverting to the factory version is not required to disable an app.
It's simple. The dev team think that by disabling, you mean you don't need that app anymore so it's of no use to keep an updated version, so they decide that the 'Disable' button will take the same place as 'Uninstall Update', resulting in less buttons in the app info page.
In actuality, you of course can disable a system app without reverting to factory. However this requires command-line operations, like adb or a terminal emulator. Type in  

pm disable com.example.foo  

and hit carriage return, the app will be disabled without reverting.
To re-enable, type in  

pm enable com.example.foo  

and hit Enter.
That's all. Pay attention that you should replace com.example.foo with the actual package name.