Why not use double quotation marks in poetry?

Because there are no double quotation marks in poetry.

Double quotation marks generally do not appear in poetry, and exclamation marks can, and it is not a big problem not to add them. The key is to see whether the words are beautiful or not.

Double quotation marks are indirect quotations, while single quotation marks are references in quotations. They can be direct quotations from characters' languages, quotations from articles or quotations from proverbs and aphorisms.