The earliest structure editor was Interlisp's original structure editor for teletypes, which worked on Lisp lists directly. Editing on teletypes is not anything like editing on video terminals - you are basically forced to edit one line at a time instead of viewing the whole file. That's how ed works. Structured editing made a lot of sense for teletypes because you did not have to worry about putting delimiters in the wrong place, which is very hard to spot when you are doing things one line at a time.
There was an Interlisp structured editor (maybe more than one? I'm not sure) written for video terminals in the 1970s. On video terminals structured editing loses a lot of its advantages over text editing and structured editors fell out of style in the 1990s.