Why does author of Elements of Style use an authoritative tone?

Strunk wrote his original manuscript in the authoritative tone of the professor speaking from the lectern, and White, in his additions, followed Strunk's lead. While the authors acknowledge that some of their views are not universally held, they go on to present those views as representing the highest standards of written English. Virtually all of the book's rules and principles, and also much of the text that supports them, are presented in imperative sentences: "Put statements in positive form"; "express coordinate ideas in similar form" (the principle of parallel construction); "revise and rewrite." Following their own advice about not weakening sentences with vague qualifiers, Strunk and White never write "try to . . ." or "it is a good idea to . . ." or "if possible . . ." Their presentation can be summed up as follows: These are the rules. Good writers follow them. A reader of The Elements of Style is likely to conclude that Professor Strunk was not in the habit of asking his students, "Are there any questions?" His rules of written English are clear and neat and not open to discussion.