FlatBuffers, Protobuf, Cap'n Proto, etc., all require an external schema configuration that you compile into a code chunk that you include into your program. Without this it is impossible to make sense of the data. In our case, the data is semi-structured and changes frequently. The prospect of maintaining a schema registry for all the data users and keeping everyone up to date and backwards compatible is enough of a burden that it was excluded.

Avro also uses schemas, but since the schema is embedded in the file it is self-describing so the reader does not need to do anything special to interpret the data. But Avro's C library is buggy and the python deserialization performance was terrible, so Avro was not selected.