It is actually diabled on purpose in quzzes. When you load more than three instances of the HTML editor on a page, it causes browser (mostly IE) to fail in one way or another.
Instructors are advised to set essay quizzes to 1 question per page, to encourage students to concentrate on one thing at a time, avoid browser issues, and force frequent save of user responses. ( text editor will lshow up then on every essay question)