Often you're forced to use it, because the infrastructure you're working with only speaks JSON (e.g. databases whose wire protocol is JSON-based, like CouchDB.)
And even if you get to pick your own infrastructure, a lot of development effort is "stolen" by JSON-focused tooling and infrastructure, such that there's very little development effort given to protocols that aren't as limited.
Consider: how many languages that offer a "batteries-included" JSON parser+generator, also offer a "batteries-included" parser+generator for e.g. ASN.1?
And even if you get to pick your own infrastructure, a lot of development effort is "stolen" by JSON-focused tooling and infrastructure, such that there's very little development effort given to protocols that aren't as limited.
Consider: how many languages that offer a "batteries-included" JSON parser+generator, also offer a "batteries-included" parser+generator for e.g. ASN.1?