Why? Because TalkTalk removed the ability for users to Add, Create and Delete mailbox addresses in July 2023 and have not restored those functions to users. TalkTalk Staff can Add and Delete mailboxes / addresses but it's only the Data Protection Team that can take on the responsibility of deletion and only after the request is made to the TalkTalk Data Protection Office and the requester is verified as the registered user.
See the GDPR help page item 'Your Right to be Forgotten / Right to Erasure '. There's a postal address, which in my view is the best way to contact the Data Protection team, but you can initiate a contact via any of the TalkTalk contact methods.
