There are third-party keyboards, but none of them are good. That's not the third-party developers' fault: the iOS keyboard API is so wimpy, developers literally cannot make a keyboard that works like the default ones.
I don't want to hunt and peck, so on iOS I mostly use the Japanese "flick" keyboard, which is much more logical, but doesn't offer English autocompletion, so it's just bad in a different way.
I really don't know why iOS doesn't support Dvorak.