Putting the format in the URL itself is much easier than having to fiddle with HTTP headers. Not to mention the ability to view API data directly within the browser is a major advantage in discoverability.
Using accept headers and browser discoverability do not have to be mutually exclusive. You can recognize browsers based on user agent and relax the requirements on accept headers.
Putting the format in the URL itself is much easier than having to fiddle with HTTP headers. Not to mention the ability to view API data directly within the browser is a major advantage in discoverability.