Before the 12th Imam (may Allah hasten his reappearance) went into the major occultation, why didn’t he explain in great detail the scope of the authority that jurists would have in his absence.

The detail of this was explained by the 11th Imam (A.S) while he prepared the
community for the concept of occultation – ghaybah – that is, the time in which his son would remain unseen and hidden from the view of people.
And, during the initial or minor period of occultation of 12th Imam (may Allah hasten his reappearance) the rules regarding Islamic taxation – Khums – and all other duties were laid open to the Muslim community by his four deputies. These were, in succession: Uthman ibn Saieed, Muhammad ibn Uthman, Hussayn ibn Rouh Nawbakhti and Ali ibn Muhammad Saimori. For over 70 years, these four were the Imam’s link to the nation of Muhammad (PBUH) the ummah – and they explained these matters in full.