The problem is that it still needs a conversion to classical mail (just now on the server instead of client side) which comes with the problem of braking signing.
I'm not quite up to date wrt. JMAP but the last time I checked (quite a while ago) their approach to "fixing" that problem was security wise fundamentally broken.
Through then most mails are not signed anyway. So for many use-cases this isn't an issue.
The normal MIME based mail format with all it's pain in header encoding, multi part bodies, language and encoding annotations etc.
EDIT: Note to be confused with Media-Types which are a (very) small part of the MIME specification and are sometimes called MIME Types. Due to their use in http they got their own spec later on (and are also redefined in the HTTP spec instead of referring to the mime spec, funnily their re-definition in the HTTP spec doesn't 100% match the specification in the MIME spec, luckily only in "don't do this every" parts).
I'm not quite up to date wrt. JMAP but the last time I checked (quite a while ago) their approach to "fixing" that problem was security wise fundamentally broken.
Through then most mails are not signed anyway. So for many use-cases this isn't an issue.