No, it's just the most widely supported protocol of that kind, so why not use it? Is there anything specific against it/how do you answer for another protocol the question of "why not HTTP, which is similar and has better support?"? (Caveat: I think you'd ideally want http/2, which isn't as widely supported)

EDIT: actually, one point: you can talk JMAP over HTTP(S) directly from a browser without a translating server.