One is clearly that you need to communicate the requesting IP deep enough into the network stack to the point where you get the DNS response (if there is one), which means there's a fair bit of work to ensure this is done everywhere;
Another is it's known to break corporate websites (https://internal.bigcorp.com/ on a public IP expecting to be able to access private IPs on their intranet), and smaller browsers are reluctant to lose corporate users by changing long-standing behaviour;
Then there's the WebRTC question: if two users on the same local network start a WebRTC call, do we want all this traffic to go to via first point where the public IP is known? For the majority of users the answer is no. And as long as that's possible, there's a (very limited) means of local communication. (But if it's limited to only things that respond to the WebRTC handshake, we're already in a much better place!)
