Ease of use
Any security product needs to be very simple to use if it’s going to become successful. If they’re not simple to use then the cost of supporting difficult products can easily outweigh any benefits from them. That’s why Voltage’s SecureMail has the minimal level of user involvement. If a person sending an email can click on the "Send Secure" button instead of the "Send" button, the can use SecureMail. There’s nothing else that they need to do.
It’s probably possible for people to use more complicated secure mail systems. President Obama probably has no difficulty using secure email on his BlackBerry, but he has a fairly large staff to configure it for him. He probably has fairly good tech support too. Similarly, generals don’t seem to mind using digital certificate from the Department of Defense’s PKI to send signed and encrypted email, but they also have a staff to take care of any problems that might occur.
People that don’t happen to be the President of the United States or a general officer still need to encrypt email, however, and they normally have to do it on their own. In these cases, ease of use is critical.
There are also probably good reasons why people simply don’t want to use secure email that takes more effort than clicking on "Send Secure" instead of "Send." It’s probably very similar to the reason that many people don’t use the latest social networking application, or whatever the trend du jour is. This may be because they just have better things to do.
When you’re young, you tend to have lots of free time, but also don’t get paid much. This means that you have the time to do what you want to do but sometimes can’t afford to do these things. You’re resource constrained, not time constrained. Not too many years later, most people find that this situation has reversed. At that point, they find that they’re married, have children and that their job now carries more responsibilities than can easily be done in an eight-hour day. At this point, there are more demands on their time than there are hours in the day, and they’re now time constrained instead of resource constrained. When that happens, learning a new security technology is now competing with dozens of other priorities for the little time that’s available.
To most people, spending the time to learn a complicated security application never becomes a high enough priority that they decide to do it. There’s always something else that’s more important. This isn’t limited to security, of course. This may also explain why you see lots of younger people using the newest social networking applications while those that a bit older often don’t get around to using them: there's often a better use of their time.