But what do you need the 8.1 ISO for? If you don't have a real key for it, you can't activate it. I don't know if the 8.0 keys are still activating at this time on a Windows 8.1 upgrade from the Technet/MSDN ISO. The Technet/MSDN ISO's issue new keys for subscribers.
If you had 8.0 with your system, then you upgrade via the store if you need to reload.
If you have 8.1 with your system, then you just install the 300mb GA updates and anything after this time and you are good to go.
Oh I dunno
1. You have to re-install
2. You bought the upgrade version and don't want to install 7, 8, then 8.1
3. You don't want to wait hours for he 8.1 update to install.
4. You want your install to be as clean as possible.
5. Because its dumb as hell for not offering a standalone offline installer.