PGP is better than the alternative
In the early days of Outlook / Netscape Navigator, they adopted S/MIME for encrypting messages.
Sadly this doomed encryption almost from the very beginning because:
1. The user interfaces for using encryption were awful, barely afterthoughts
2. S/MIME uses certificates with rsa asymmetric encryption and it was SLOW
3. Obtaining a new cert/key was a massive pain in the arse and usually involved paying money. A tax on security.
4. Keys expired every year, compounding the pain.
So it was a garbage implementation of a garbage crypto mechanism.
By contrast most PGP extensions to email apps were relatively sane by comparison - create a key for free, use it as long as you like, add other people to your web of trust. It all fits relatively naturally with email but as extensions the experience never felt fully integrated (despite being easier than the built-in crypto) so few people bothered. And using crypto always felt like standing above the parapets - you must be up to something to be using crypto rather than it being the default for everyone.
And these days with webmail, any chance for secure encryption by default is long gone. Even if the transit of email is secured, even if the viewing of the email is secured, the actual email itself isn't. Google (for example) can and do read emails, ostensibly for beign reasons, e.g. so they can remind you about your upcoming flight or whatnot, but who knows what else they do or who else they allow to see it.