[The Wisdom Behind the Variation of the Evidences of Obligatory Rulings]

Answer (and Allah is the Grantor of success): The wisdom behind that includes:
1- Increasing the burden of responsibility, for reward is in proportion to hardship.
2- Trial and testing—“the believers halt at doubtful matters.”
3- Manifesting the virtue of the scholars and distinguishing their ranks.
4- Clarifying that the obligation to know some rulings is general, while the obligation to know others is specific.
5- Indicating the difference in importance between some rulings and others, for that varies according to the strength or weakness of the evidence.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3