An Outline of the History of Restriction on Ijtihad
It is a fact that all Shi'i 'ulama' reject `ijtihad' in the sense of acting and issuing fatwa (legal opinion; pl. fatawa) on the basis of ra'y (subjective judgment), istihsan and qiyas (analogical reasoning), a which are prevalent among the Ahl al-Sunnah and considered valid by them. This position is based on what has been received from their Imams, may peace be upon them, concerning the invalidity of qiyas and the unreliability of ra'y and istihsan. Hence, ijtihad in this sense is invalid in their opinion, so much so that the works of Ibn al-Junayd, one of their earliest fuqaha' and one of the two earliest among them, have been set aside because he has been accused of practising qiyas. 
[The Causes of Variant Fatawa:]
From what we have mentioned, the reason behind differences of opinion among Shi' 'ulama' themselves regarding issues of law becomes obvious. Its basic cause lies in either the different opinions regarding the acceptability of a particular text, or the degree to which it is found convincing by someone, or the different degrees of sharpness, intelligence and perspicacity of minds in dealing with the issues and the texts involved.
[Ijtihad Among the Ahl al-Sunnah:]
As to the other Muslims who retracted from the oath of allegiance to Amir al-Mu'minin `Ali, may peace be upon him, they came to hold that the Prophet, may God bless him and his Household, neither appointed any successor nor made any will for anyone to exercise wilayah over Muslims after his demise. Indeed [they maintain] he refrained from making any will in order to leave the community fully free to determine a successor and ruler itself. Hence it may choose from among the community any person it desires and concurs upon.
Similarly, [they held that] he designated no specific person as an authority in matters of the law of Islam. Rather, he left [the exposition of] the Islamic Shari'ah after himself to the generality of his contemporaries (ashab) because all of them possessed the quality of `adalah  (credibility) and the capacity for ijtihad, alleging that [the Prophet had declared]:  (`My Companions are like stars, you will be guided if you follow anyone of them').
Consequently, for these Muslims the basis of practice in matters of law was the authority of the Companions and their fatwds, irrespective of whether the fatwa was based on a tradition of the Prophet, may peace and God's blessings be upon him and his Household, which the Companion had heard from him or was the result of his own ijtihad and ra'y. This was so because the Companions were considered unerring (musiban) in their ijtihad, as is elucidated in their books on jurisprudence (usul).
It is not our present concern to debate the validity of these claims or to provide a proof that this tradition is a forgery ascribed to the Prophet, may peace and God's blessings be upon him and his Household. In this regard, the 'Allamah of India [Mir Hamid Husayn], in the second of the two volumes of his book 'Abaqat al-anwar on Hadith al-Thaqalayn, in some 250 large-size pages of his book where he cites many evidences from the works of some most eminent Sunni 'ulama' who affirm its fabricated character, has cited seventy reasons invalidating the tradition. This study may be referred to on this issue whose elaboration is outside the scope of our discussion about restriction on the number of schools of law and the question pertaining to it.
[Sources on the Beginnings of Proliferation of the Schools:]