As to why some of the caliphs...
As to why some of the caliphs, who apparently wished to show devotion to the Prophets family (p.b.u.h.), turned over Fadak to the grand children of Zaid ibn Ali, for example, or other unknown persons as representatives of Bani Hashem instead of the Imams themselves, there are two probable reason: The guardian Imams (p.b.u.h.) were never willing to accept Fadak because at that time this had more of a material nature than a meaningful one, and maybe gave the impression of attachment to the world instead of spiritual distinctions.
By another interpretation, accepting it under those conditions was unworthy of the guardian Imams (p.b.u.h.). In addition to that, it would also tie their hands in their struggle against the oppressing caliphs. This was because each time they wished to rise up against them, they had to give up Fadak. (Just like in the affair of the repossession of Fadak by “Abu jaa’far” Caliph Abbasi from “Bani al-Hassan”.
It has been reported that after the uprising of some of them against the caliph’s government, he took Fadak from all of them.) The oppressive Caliphs also preferred that the material resources of the Imams not be increased. Just as it is well known in the celebrated story of Haroon that when he came to Medina he showed unusual respect for Imam Moosa ben Jaafar (p.b.u.h.), in a way that was surprising to his son Mamoon.
However, when it came time for gift giving, the gift that he sent to the Imam (p.b.u.h.) was unusually meager. Mamoon was mystified by this matter and upon questioning his father for the reason behind it, his father gave an answer which was in brief something like; “We must not do something to help them gain power and tomorrow rise up against us!” Previous…