The vast majority of people don't seek out or even buy alternatives. The vast majority is incapable of configuring their own mail client and usually don't have

The vast majority of people don't seek out or even buy alternatives. The vast majority is incapable of configuring their own mail client and usually don't have enough mail to require anything beyond what any client offers. The people that do on the other hand and are therefore part of the potential audience are much more likely to sign and encrypt emails.