The JSON Wire Protocol is on it's way to becoming accepted as a W3C standard. This means that moving forward, the vendors behind browsers will have much more incentive to implement and maintain implementations for their browsers. Some browsers such as Mozilla and Microsoft have already taken control of the drivers used by Firefox and Edge. I heard google has declared intent to take over chromedriver, but theres no real ETA for that. Each vender implements the wire protocol their own way.
One reason why browsers do not currently implement this protocol directly is that because it's not a universally accepted standard yet, and thus, venders simply don't know if the effort they put into making and maintaining it will be sustainable.
Historically (I'm not sure if this is still the case), Firefox's implementation was a firefox plugin that would be installed in a new, disposable profile that ran a tiny http server that would receive commands and perform javascript operations accordingly.
The JSON wire protocol uses JSON as the name implies to package commands and uses HTTP to send those messages to whatever http server is ready to recieve them. This gives near universal support as most technologies these days utilize and can understand these protocols.
https://www.w3.org/TR/webdriver/
Hope this helps!