I think it's related to how atlassian develops their products - they acquire then. Jira, confluence and Trello have three different editors because they're three different products developed by three different companies.
And yet, they are trying to move Trello to the terrible rich text editor like Confluence. So far you can still opt out and send a "wtf are you doing?" feedback about the new editor, but I'm not sure how long that will last.
