I worked on a pull request to implement it in Terminator a long time ago, but eventually gave up. Control mode was contributed to tmux by the author of iTerm2, and if you explore the iTerm2 codebase you can definitely see that they're very tied together in terms of implementation choices and general design. It's not that simple.
In order to have two-way synchronization, your terminal needs to not only have an architecture where you can add a background thread that can control the "main" terminal interface, but also be flexible enough to properly size all windows and panes, and match font sizing. If your terminal supports different font sizes for every pane/split, those have to be turned off in tmux mode. There's all kinds of corner cases.