Encrypted messaging has been a complete failure; there is no need to single out email. I suspect the reason is more or less the same in all cases. Users have not been provided with a conceptual framework that would allow them to use the tools in a reasonable way. If the US federal government can come up with, and promote such a framework the world would become a different place.
BTW, the linked article is mostly based on misconceptions:
Can you elaborate on why you see it this way? WhatsApp has been wildly successful, my very non-technical in-laws use Signal for their family's conversations, and other messaging platforms are jumping on the bandwagon.
As far as I can tell, if we lose encrypted messaging at this point, it will be due to government action or corporate rug-pulling, not because it failed to catch on. Whereas encrypted email really hasn't caught on anywhere.