Why? I have been using mailbox.org for a few years and I am very happy with it.
Not OP, but the Proton UX / UI is far superior to Mailbox.org - which i personally find hard to navigate and disorganized...
i pay for mailbox.org but i never use the webmailer haha, they support IMAP/SMTP
I agree the web app is not amazing, I use a native mail application.
Proton not supporting third party apps and requiring minimum Android 10 to launch their app can keep their app for themselves, clearly they don't want more users.
I'm still on fastmail because I use *@domain.name and Proton only supports a very limited number of emailaddresses, and only 3(!) domains. Sadly.
I like the UI simplicity of posteo. They still have quite some advanced features like encrypted address books and more, with good documentation on the implications. No regrets so far, using it since beginning of the year or so.