One difference is that Signal and Whatsapp are synchronous so they are able to use deniable authentication. That's in contrast with asynchronous email where sig

One difference is that Signal and Whatsapp are synchronous so they are able to use deniable authentication. That's in contrast with asynchronous email where signing a message can then be proven to have come from the sender to a third party - not always desirable. https://en.wikipedia.org/wiki/Deniable_authentication https://en.wikipedia.org/wiki/Deniable_authentication etc. Perhaps you can do it with asynchronous protocols too, though it seems harder.