Preventing intermediate routers from screwing with the protocol is a higher priority. Microsoft already has an embrace-extend-extinguish proposal aimed at mobile networks, and that could easily make the protocol unusable. The websocket handshake required a ridiculous number of iterations to not trigger bugs in middleboxes. As a nice side effect, encryption everywhere removes some of the payoff for traffic interception by ISPs, like BT's attempted deployment of Phorm adware.