لأن الحكم في الأصل لم يعقل معناه فاستحال تعديته إلى الفرع
If the original ruling's effective reason cannot be identified, there is no established cause by which to extend it.
Certain rulings attach to special cases whose underlying cause is not independently clear. The classical question of ablution with date infusion involves particular reports and juristic disagreement; extending that result to every other liquid would require an established shared cause.
Saying 'both are liquids' is insufficient. A special legal ruling and a transferable legal cause are different things.