OpenPGP end-to-end encryption has to be done client-side, and the keys have to be stored client-side. This is hard to do given the current trend of using webmails. Besides, using OpenPGP means that you have to store a copy of your key on all the machines that you use to check email, and people usually do not know how to do this without entrusting a third party (Google, Dropbox, Apple, etc.) with the information.