Which reputable email service providers beside Fastmail (who developed the spec) have mature support for JMAP? Thunderbird’s implementation is far from being mature, in the same way Thunderbirds EWS implementation is.
So, unless more reputable email providers support JMAP, I don’t see lots of people really wanting it. At least Google, Microsoft, and just about every other email provider out there support IMAP, regardless of whatever API scheme they put on top of the HTTP(s) protocol for additional access methods. I do like how JMAP includes Calendar and Contact support, but even CalDAV and CardDAV is more widely supported (GMail for example), especially in iOS/MacOS mail, contact, and calendar apps.
The problem is not eM Client not supporting JMAP, it is that most email service providers not finding it worth the effort to support JMAP. When that changes then it will make more sense for eM Client to support JMAP. Wake me up when Google and Office365 support JMAP in the same way they support IMAP or their native APIs.