* Email defaults and fails open to plaintext. Modern secure messengers only transmit securely. Even modern secure websites take pains, including extensions that had to be made to the HTTP protocol, to prevent traffic from being sent unencrypted. No such mechanism exists in email.

* Email reveals large amounts of metadata, including message content (which is what a Subject: line is), which is a characteristic shared by no secure messaging system. The graph of who talks to who and when is immensely important, and the best secure messengers (Signal, in particular) go through trouble to obscure it.

* Modern secure messengers are forward-secure, so that a compromised key doesn't allow attackers to retroactively decrypt old messages. Since messaging traffic between selected peers is almost always low-volume, any reasonable threat model assumes every piece of ciphertext exchanged between peers has been archived by adversaries. With PGP, a key compromise at any point in time reveals the contents of every one of those messages. The best secure messaging systems are not only forward secure but also have a notion of "future secrecy" allowing them to recover from ephemeral key loss.

* Email clients are inhospitable to cryptography; in particular, many of key features of clients (like search) rely on having a local archive of plaintext. The UX in email clients begs users for catastrophic failure, like accidentally top-posting a reply to an encrypted messages with an unencrypted message.

All four of these problems are, in isolation, total dealbreakers for serious secure messaging.

SMTP email is janky and outmoded and it's a little amazing that we all use it as much as we do; it's long overdue for a replacement. If you need to communicate securely with someone else, you should do so with a secure messenger like Signal, not with PGP and email.