It also follows unix conventions in option placement and syntax, so there's never any cognitive hurdle to cross when the question arises where to put headers, where to put form data, where to put query string data, how to handle the case of query string data being supplied using overlapping methods and what the form data is encoded in.
It also doesn't force me to write json in yet another syntax that's command-line friendly. Instead I can just write json.
Yes. HTTPie looks nicer, but the wide availability of curl and the much more predictable syntax make it a better tool for my taste.
If I need formatted JSON output, I can always pipe the result in to jq.