It isn't because GnuPG doesn't work well or because it is too hard to use that people don't use it. They don't use it because other people don't use it.

Why do people use Facebook instead of Diaspora? It's not because Facebook is better, works well, or is easier to use... It's because other people use Facebook.

Why do people use Skype instead of XMPP? It's not because Skype is better, works well, or is easier to use... It's because other people use Skype.

Why do people use WhatsApp instead of GnuPG or OTR? It's not because WhatsApp is better, works well, or is easier to use... It's because other people use WhatsApp.

It's always the same standard chicken and egg adoption problem. As shown in the article, GnuPG is used for package signing, commit signing, file encryption, and all sorts of things that don't involve many people. But when it's about communication, you can only use GnuPG if the people you want to communicate with use GnuPG too... and thus the adoption problem applies.

And the reason people initially used Facebook, Skype, and WhatsApp is not that they were easier to use or better. It's advertising. Notice how all of these are proprietary software made by companies with the means to advertise their software? You can bet people would use GnuPG, Diaspora, and XMPP if they had been advertised by companies like Facebook and Microsoft.

So if we want people to use open source software and open protocols instead of living in walled gardens, we need to advertise them. Advertising works.