Why can't I use my iCloud email address as my Apple ID

If you had an existing (non-apple) apple id before icloud, apple gave people the ability to have an icloud (email) account attached to it. By signing into an icloud enabled device, and checking off mail, you are given the option to create an @icloud address (formerly an @me.com one), that is associated with that apple id. This allowed people to continue to use the apple id that had all their purchases, and have an @icloud address. You can sign into any apple services using this address and pretty much ignore the originating non-apple address if you want. If that originating address is no longer valid... well, that's not really apples fault. You can maintain it, or create a new non-apple address to change it to (only reason you'd really need to do this is for the sake of rescue emails).

If you don't have an apple id, and wanted to use icloud, you can create an icloud apple id by signing into an icloud enabled device. This is an apple id in its own right, and therefore cannot be used to change the address of an existing non-apple id.

This is essentially how it works, and is pretty easy to understand.

Now, what you guys are wanting is a new feature, which DOES make sense. I'm with you guys on this. You want to be able to remove the non-apple address from your accounts, and just use the @icloud address permanently, without a non-apple address. After all, new icloud signups can do this...

Anyone wanting this new feature should request it at apple.com/feedback.