It has the advantage of not relying on the Certificate Authority system, and not requiring a full web stack (which some email clients and servers wouldn't want to open themselves up to). DNSSEC also allows Authenticated Denial of Existence, so that unavailability of a server isn't treated as meaning the user doesn't have a key in the directory.
For reference, this technology is also already supported by GnuPG and Posteo.de:
https://tools.ietf.org/html/rfc7929
It has the advantage of not relying on the Certificate Authority system, and not requiring a full web stack (which some email clients and servers wouldn't want to open themselves up to). DNSSEC also allows Authenticated Denial of Existence, so that unavailability of a server isn't treated as meaning the user doesn't have a key in the directory.
For reference, this technology is also already supported by GnuPG and Posteo.de:
https://posteo.de/en/blog/new-posteo-public-key-directory