I think their goal is to have transport layer be a pluggable subsystem, divorced from their request-response protocol.
They even essentially say that HTTP is not necessary to their protocol, so I think it's pretty apparent that HTTP is a utilitarian choice, but they're not trying to build email-over-http.
Re:JMAP: A modern, open email protocol
by specialist on Tuesday May 07, 2019 @05:12AM
(#19847716)
”pluggable subsystem”
SOAP had same rationalization.
They're not just doing IMAP over http, they invented their own error bodies, schema's , negotiation and RPC.