The nickname was changeable in A14 or A15. And it was abused very often (like spamming with nickname changes, impersonating other people, ...) So that feature was removed on the request of the players.
Also, requiring the email address would make the registration process a lot harder (enter an extra field, wait for confirmation mail, click confirmation link, log in to the game again). While I thought you just wanted the registration process to be easier.