Bootstrap is over rated and why use a third party framework for ACP when they already have a user friendly and near perfect ACP? What is your reason for wanting it?
Then you can hire a developer/designer to modify the ACP for you. Suggesting or wanting something in the core primarily for personal reasons/liking isn't going to work out for a product that is sold to thousands.
Bootstrap also adds lot of fluff, what you can do in bootstrap with probably 16 lines of code and with many pointless <divs> you can do it yourself in a much better manner, it isn't all about looks.
XenForo's current ACP is the best out there compared to other products, in terms of friendliness and functionality.