Yeah, it’s a shame. IMAP is not a great protocol (to say the least), but jmap is nowhere near critical mass. At present, its not worth the effort to implement for pretty much anyone, so it’s only idealists like Fastmail who’ve done so anyway.
Worth mentioning that Fastmail use an open source backend: Cyrus[0] - which started adding JMAP support in 2015 (though I suspect that was Fastmail devs contributing the feature).
Their primary targets are Microsoft Outlook, Apple Mail and Thunderbird, none of which speak JMAP at present. So Proton has very little to gain from implementing JMAP, since only niche clients support it at this point.
Classic chicken and egg problem, unfortunately.
Still hope at least both will add push notification to 3th party apps. And a BIG hope for https://stalw.art/