Previous discussion, leading with a comment from one of the protobuf authors: https://news.ycombinator.com/item?id=18188519
kentonv was willing to engage on the points and came out much more reasonable in the whole thing.
Sanjay, Jeff, and Kenton are probably the three best to answer such questions.
Presumably the top few concerns for protos are wire performance (decode/encode speed and cost, wire size), compatibility for changes (what this suggestion just totally breaks), and cross language usability.
Some other tradeoffs might be non-wire perf (I believe protos beat flatbuffers here, at the cost of worse on wire perf), but it's not clear that that was intentional.