Every candidate has their own eCAS account setup already. The login gets 'activated' once there is an active application.
Once AOR is received, the application is active and you can log into eCAS to see the updates.
Emails or mails are sent only when an action is required from the candidate. eg Fingerprints, interview/test date, oath date etc
Since 'InProcess' and 'Decision made' updates dont require any action from candidate (and is purely informational), no notifications (via email/mail) are sent
Hope this made sense