The evolving schema is much more attractive than a bunch of plain text HTTP headers when you want to communicate additional metadata with the file download/upload.

For example, there are common metadata such as the digest (hash) of the blob, the compression algorithm, the base compression dictionary, whether Reed-Solomon is applicable or not, etc...

And like others have pointed out, having existing grpc infrastructure in place definitely helps using it a lot easier.

But yeah, it's a tradeoff.