The "other parsers" in stdlib are gob and encoding/xml.

Gob is a format made for Go, so it does not have a field naming convention mismatch; what's on wire is the Go convention.

encoding/xml isn't really structured as a "convert between equivalent-shaped structs and a message" utility, its struct tags are more like a query language, like a simpler cousin of XPath. Most real-world code using it does things like

    type Person struct {
        FirstName string   `xml:"name>first"`
        LastName  string   `xml:"name>last"`
    }

Where as with JSON there was a clear desire to have

    {"email": "jdoe@example.com", "name": "John Doe"}

parse with as little noise as possible:

    type Person struct {
        Email string
        Name  string
    }