Why doesn't GoodCrypto support S/MIME?

S/MIME is more about tracking. PGP is about privacy.

PGP has a strong record of protecting users' privacy. So GoodCrypto uses PGP.

Background: In the early 1990s, the PEM group tried to develop an open standard which would protect everyone's email privacy. NSA pushed the standard so hard they offered free @nsa.gov email addresses to members. Why? PEM was an email tracking system. The PEM charter said that tracking was required, but confidentiality was optional. When the group would not accept this low standard, that group was shut down and replaced by an invitation-only group that developed S/MIME.

Additionally, S/MIME relies on centralized certificate authorities which are easy targets for mass surveillance. Certificate authorities can issue both the public and private components which also weakens security.