I don't know but if I had to guess.

1. Google uses protobufs everywhere, so having something that behaves equivalently is very valuable. For example in protobuf renaming fields is safe, so if they used field names in the JSON it would be protobuf incompatible.

2. It is usually more efficient because you don't send field names. (Unless the struct is very sparse it is probably smaller on the wire, serialized JS usage may be harder to evaluate since JS engines are probably more optimized for structs than heterogeneous arrays).

3. Presumably the ability to use the native JSON parsing is beneficial over a binary parser in many cases (smaller code size and probably faster until the code gets very hot and JITed).