I have thought about this a lot, but came to the conclusion, that people are most likely to write tools if the format is easy to parse and construct in many programming language. I think it's hard to find an encoding that suits this criteria better than JSON.
It also has the benefit of being single-lined, since newline characters are encoded, which is necessary for a line based format. This, in turn, allows the use of many UNIX tools, like grep.
However, it certainly is not the most compact format, when encoding large amounts of binary data. Gzipping for storage will alleviate the overhead somewhat. Overall, dealing with large amounts of binary data in web application testing seems like a less common use case, so I felt like I shouldn't put too much focus on it.
It also has the benefit of being single-lined, since newline characters are encoded, which is necessary for a line based format. This, in turn, allows the use of many UNIX tools, like grep.
However, it certainly is not the most compact format, when encoding large amounts of binary data. Gzipping for storage will alleviate the overhead somewhat. Overall, dealing with large amounts of binary data in web application testing seems like a less common use case, so I felt like I shouldn't put too much focus on it.