JMAP was developed by the guys that run FastMail (who are the primary developers of the open source email server Cyrus-IMAP). It’s easier to implement and more performant.
Side note, Cyrus is a pretty amazing mail server. It doesn’t get much love here, but it’s bombproof, fast, supports multi-node clusters, IMAP, NNTP, CalDAV, CardDAV and more that I’m forgetting. It’s just a bit old school as far as configuring goes.