Strong email encryption has been freely available since 1991. Seventeen years later, essentially nobody uses it, and the usual explanation is that the software is too hard.
The software is hard. That is not the reason.
The reason is that encrypted mail is worth nothing to one person. If I generate a key, protect it properly, and publish it, I have achieved precisely no security, because every message I send goes to somebody who has not done any of that. My effort produces a benefit only when the recipient has made the same effort, and their effort only pays off if their correspondents have too.
Almost every technology that spread offers something to the first person who adopts it. A spreadsheet is useful alone. A telephone is not, but a telephone requires the other party to do nothing except own one, which is a far lower bar than asking them to understand key pairs.
So the adoption curve never starts, and blaming the interface misidentifies which part is stuck.
The failure mode does not help either. Lose the key and you have not lost access to a service, which is recoverable by asking somebody. You have lost the contents of your own archive, permanently, in a way no support desk can undo. That is an unusual risk to ask an ordinary person to carry in exchange for a benefit they cannot perceive.
There is a further problem that gets much less attention than it deserves, which is that encrypting the message body leaves the envelope untouched.
Who wrote to whom, at what hour, how often, in what pattern, with what subject line, from what address, travels in clear text and must, because the system needs it to deliver anything. For a great many purposes that is the sensitive part. Knowing that a person exchanged eleven messages with a solicitor at two in the morning tells you nearly everything worth knowing, and the contents would only confirm it.
So the mature technology protects the part people imagine matters and cannot protect the part that frequently matters more.
If this is ever solved it will be by encryption happening automatically, invisibly, between systems that negotiate it without anybody choosing anything. Nobody sells that, because there is no product in a feature the customer never sees.