[jgraham]

There's an issue open for this on the WebDriver BiDi issue tracker.

We started with WebSockets because that supports more use cases (e.g. automating a remote device such as a mobile browser) and because building on the existing infrastructure makes specification easier.

It's also true that there are reasons to prefer other transports such as unix domain sockets when you have the browser and the client on the same machine. So my guess is that we're quite likely to add support for this to the specification (although of course there may be concerns I haven't considered that get raised during discussions).