It's called "sadCamelCase", as opposed to "HappyCamelCase" ;)

Co-author of proto3 here. The reasons we chose lowerCamelCase are compatibility with Google's REST APIs (like Gmail API) and readability for users who work with JSON output directly without client library. API designers should not define confusing data schemas, let alone allow collisions. Most proto messages have small number of fields, avoiding name collision is a trivial for an API designer.

I think this may be to maintain style conventions with JavaScript and previous versions of Google Cloud APIs which were all based in lowerCamelCase (not 100% sure though, don't quote me on this).

A benefit of this decision is that if you create JSON manually in JavaScript (ie. without a protobuf library) your JSON objects will match JavaScript conventions.