One of the primary reasons for choosing S/MIME was its ubiquity. Most people don’t use it, but almost all mail applications at least support it, especially clients developed in the last decade. It’s curious that PGP hasn’t managed a similar feat; the fact its competing with a RFC standard hasn’t stopped similar quasi-standards from winning in the past.
Meanwhile, this is helpful:
Opaque-Signed and Encrypted S/MIME messages: When S/MIME messages are received in Windows 8 Mail, it displays an email item with a message body that begins with “This encrypted message can’t be displayed.”
I don’t think anyone here (or anyone in their right mind) uses Windows 8 Mail, but keeping a link to OpaqueMail for future reference:
Windows 8’s Mail App does not natively support S/MIME. However, the OpaqueMail Proxy can add S/MIME protection to Windows 8 Mail.
Windows 8’s Mail App can be configured to use the OpaqueMail Proxy. It includes an option to allow “loopback” network traffic. In addition to signing and encrypting outgoing messages, the OpaqueMail Proxy can automatically import certificates from inbound messages.