Because Imam Baqir (A.
Because Imam Baqir (A.S.) is the first grandson of Imam Husain (A.S.) and the second grandson of Imam Hasan (A.S.). His mother was Fatema, the daughter of Imam Hasan (A.S.). Hence Imam Mahdi (A.S.) is the son of Imam Hasan (A.S.) as well as Imam Husain (A.S.). Hilali relates that once the Holy Prophet (S.A.) consoled his daughter Hazrat Fatema (S.A.) in these words : The two sons of this nation are your sons Hasan and Husain, who descend from me. They are the chiefs of the youth of Paradise.
By Allah, their parents are superior to them. 0 'Fatema ! by Allah the Mahdi of this nation will be the descendant of both of them." (d) Keeping these facts in mind that it is a lone tradition which is contradictory to other several continuous traditions, also there may he a possibility of error in script. This becomes crystal clear that it is not an authentic tradition particularly when the historical facts are contrary to the tradition.
The reason is that those who are attributed as Mahdi, among them is the grandson ofImam Hasan (A.S.), Moham-mad bin Abdullah known as 'Nafse-Zakiyya. Bani Abhas's campaign attributed him to be Mahdi so as to earn the sympathy of masses and take undue advantage of this belief which is deep-rooted in their hearts and thus topple the reign of Bani Umayya.
They publicized and promoted his name and first thing they did was that they added 'his father's name is same as that of my father (as it was hinted in the discussion on third tradition), since the name of this 'Mahdi's' father was Abdullah. Then for dispelling further doubts they falsely claimed that Mahdi is (lie descendant of Imam Hasan (A.S.), because Moham-mad bin Abdullah descended from the lineage of Imam Hasan (A.S.).
Then for consolidating their position they fabricated another tradition from the mouth of Abu Huraira that there was a stammering in the speech of Imam Mahdi (A.S.) (Ref. Maqatilut-Talebin) because Mohammad bin Ahdullah had stammering in his tongue. Finally Bani Abbas succeeded in grabbing power, by deriving undue advantage from this belief and intimidating their masses.
When they got absolute control then they killed the same Mahdi and his father and again spread the news that Nafse-Zakiyyah who would be killed before the Mahdi is slained and the reappearance of Mahdi has drawn nigh. Imam Jafar Sadiq (A.S.) had predicted about each and every event long back. (Maqatilut-Talebeen, Pg. 141-142, Al-Adabus-Sul-taniyyah Fakhri Pg.