Designing a serialisation format has a lot similarities with designing a programming language: data types, declarative/ease-of-use versus full control, abstractions, static versus dynamic memory areas, optimising representations etc. There is definitely room for a variety of formats depending on the use cases; and many are unhappy with the current incumbent: https://reasonablypolymorphic.com/blog/protos-are-wrong/

But yes, it's a lot of work and they probably should document what the trade-offs are that they have made.