Personally, I think it's obvious why: for the same reason SSL is widespread and why Microsoft Windows remains popular even after horrendous security incidents. It's because people are inherently lazy. They are willing to "live" with various risks/abuse for the sake of not having to click two more buttons (what's more or less involved with using PGP in Outlook).
For cryptography use to become widespread, it's going to have to be transparent to the average user. Even a minimal setup requirement will cause most people to avoid using the technology.