R. Ya'akov Kamenetsky, Emes Le-Ya'akov al ha-Torah, p. 186 n. 20:
The rule that "We do not ask questions on a derush" is not due to the unimportance of derush in relation to peshat. Rather, peshat is an explanation of the verse itself and therefore one can challenge it. However, the intent of a derush is only to present an idea or an innovation from himself and only to attach it to a verse. If you refute [the connection] from this verse, he will attach it to another. Therefore, it is not appropriate to challenge it. Quite the opposite; the main point of the idea or the innovation is like the "it seems to me" of the Rishonim, that one cannot dispute because it comes from the strength of the Da'as Torah of the innovator and not from the text itself.