Because it is very likely the pagination library should also be just using functions instead of a plug. I would rather do query |> Repo.paginate(params) or query |> paginate(Repo) than the plug or decorator approaches.
I can’t say about other languages but the fact validations in Ecto are not decorators makes them extremely easy to compose. If you need conditional validations, dynamic parameters, etc, you can just write Elixir code instead of finding the proper decorator incantation.
When it comes to composition and flow-control, there is no decorator that is going to be simpler and more readable than pattern matching, with and the |> operator (for the simplest cases).
