It's probably more the layers and layers of bureaucracy they have to go through to push any changes. They probably need at least 3 meetings, and several people to sign off on it.
Charitably, there's (e.g.) accessibility issues that need to be addressed on "new" features like the menus, and they need to be sure they're not going to be reamed for the new feature in the same way they are for this. Also testing. Several weeks is still high but not ridiculous.
My (entirely speculation) suspicion is that the old editor had a lot of tech debt, that the team was excited to delete, and since they're now having to put it back, having to make it compatible with the new editor.
My (entirely speculation) suspicion is that the old editor had a lot of tech debt, that the team was excited to delete, and since they're now having to put it back, having to make it compatible with the new editor.