I don't have any personal knowledge of this but if I were to recklessly speculate, I would wager "deleting stuff is hard". The typical concern is data loss, so to delete something completely you need to unwind your entire data loss prevention stack. Delete a file, delete a row from a database, wipe a cache from another server, scrub any ZFS-style filesystem versioning of data, mirror the changes to any off-site on-line backups and scrub everything there, retrieve any off-site off-line backups and wipe those...

It's a non-trivial engineering task. If you want an email service which can readily delete your email, you need to find one with non-redundant, completely un-mirrored databases, fragile filesystems, and no backups. But that is not what most people are clamoring for in a service provider.