PGP is quite difficult to use by most people, and it doesn't even support forward secrecy, which is a huge weakness. It will never be used by more than a core group of highly technical, which is maybe less than 0.01% of the population.
If we're to push end-to-end encryption to the masses, then we ought to try to get forward secrecy in it, and it should be quite invisible to the user.
That's not to say that ProtonMail is getting it right, but it's at least one of the few that are striving to move in that direction.
Relevant post from Moxie from Open Whisper Systems: