A subject-indexed curriculum benefits from placing all items of one kind in a single chapter. But the Qur’an is not merely a technical handbook organized by subject. Creed, guidance, law, reminder, and argument may appear together because each can support the effect of the others.
An imagined arrangement: every legal ruling first, then every historical account, then all verses about the Hereafter. Such an arrangement could separate an address from the occasion in which its warning or evidence is needed.
The distinction: thematic classification is a useful study aid; the Qur’anic presentation also responds to the wisdom of address, the condition of listeners, and the intended effect.