It's easy for me and it's easy for you - it's even easy to use once it's set up (assuming they are vigilant). But if I told my (very non-geek) girlfriend to encrypt her e-mails, she would have no clue on where to start. I could certainly help her but the problem is that not everyone has someone to ask or would even care enough to do so (obvious, since most people don't encrypt their email).
I definitely agree that everything should be encrypted, it has a great deal of benefits (aside from my opinion that cryptography is just fascinating). It's problematic though, since most people don't think that way - now we're back at square one, how am I supposed to send an encrypted e-mail to someone without a public key? Even if they had one, we still run into some problems with people not paying attention to what they are doing (did they verify that the fingerprint I gave them matched before they trusted my public key? Not likely).
I think computer security in general is far removed from many people's minds outside of paying their 40$/yr to Symantec. E-Mail encryption? They simply don't care.