Why does Android require you to revert back to factory version in order to disable a system app? In Android, you can go to the App info page and do things like see how much data storage it's using, force stop the app, and uninstall it. If the app isn't installed via the Play Store or APK (ie, it's installed in the /system partition), the Uninstall button is replaced by the Disable button.

Why does the Disable button require the app to be rolled back to the factory version before it will disable it?
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.