If you go to any Stack Exchange chatroom site (e.g. Stack Exchange Chat) the top bar is different than the regular pages. This should be changed to the 'modern' design; if not possible, could you provide a specific reason?
Chat literally hasn't changed at all from early on, and SE doesn't/hasn't had any developers working on major improvements to it in years.
So the specific reason is it's not a priority, and if there's engineering and development resources put towards chat, it'll probably be in line with the rest of the network, but there aren't.