If you start a comment out with "not strictly [related to linked article]" and hijack the discussion by plugging a website it seems like the decent thing to do is mention that you are the developer behind said website. I think this is even more important when you include so much <pre> text that the other comments are pushed so far down the page.
Maybe I'm wrong, but httpie appears to be targetting http transactions only, while curl supports multiple protocols (ftp, etc.).
curl's syntax is also baked into various Web Browser network debuggers. Both Chrome and Firefox's network transactions display have a "output to curl" option to allow you to recreate the network transaction exactly.
That is true, curl has some additional protocol features and then of course it being a standard part of most *nix OSes makes it easy to understand why Chrome and Firefox both have the copy as curl option. I still use curl regularly but for any pure http(s) stuff I use httpie because of its improved user experience.
I'd guess it has something to do with curl being available everywhere and httpie requiring installation. Oftentimes you can't enforce that your preferred alternative is deployed where you'd like to run scripts, so http->curl.
Confessions. I know I should use curl but I never do and just use wget and think curl is just a one year newer wget :(
curl is better tool than wget but I used wget first and never changed. I do use aria2c more now for downloading but I know curl is a great tool I don't use.