I'd rather explain this with necessity to learn concepts of public key cryptography, which takes time and efforts to understand. Most of people don't know about that and are not interested to know that. Interface is secondary issue: it won't take a lot of time to setup your keys given you know crypto.
My girlfriend have successfully passed the EFF's guide to PGP in half an hour (with a bit of my guidance), coping well with some inconsistences of the guide. https://ssd.eff.org/en/module/how-use-pgp-windows
There's "people who are happy reading about maths for half an hour before installing and configuring PGP" and "people who want to download an app as simple as Facebook messenger" and there's very little intersection between them.
I'd rather explain this with necessity to learn concepts of public key cryptography, which takes time and efforts to understand. Most of people don't know about that and are not interested to know that. Interface is secondary issue: it won't take a lot of time to setup your keys given you know crypto.
My girlfriend have successfully passed the EFF's guide to PGP in half an hour (with a bit of my guidance), coping well with some inconsistences of the guide. https://ssd.eff.org/en/module/how-use-pgp-windows