An Outline of the History of Restriction on Ijtihad
However, there is no doubt that the Companions became authority on religious issues after the demise of the Prophet, may peace and God's blessings be upon him and his Household. They went to different Islamic lands where they settled, teaching the Qur'an and the ahkam.
After the death of the Prophet, may peace and God's blessings be upon him and his Household, the Companions dispersed in different lands and only some of them remained with Abu Bakr at Madinah. Abu Bakr used to adjudicate on the basis of what he knew of the Qur'an and the Sunnah, and when he had nothing to lean upon he would refer to the Companions who were present. When the Companions too knew nothing in that regard, he resorted to ijtihad for ascertaining the hukm. 
Similarly, every Companion who arrived in a particular town used to practise ijtihad in matters concerning which he did not find anything in the Qur'an and the Sunnah.
Al-Maqrizi observes that when Abu Bakr died and other territories were conquered during the period of 'Umar and after him, the dispersion of the Companions increased further and the governor of each province would practise ijtihad even if no Companion were present.
This remark is explicit that the governors of provinces were considered an authority even if they were not Companions (Ashab).
[The Cause of Divergent Fatwas:]
The reasons behind divergent fatwas of the Ashab are given at length by al-Maqrizi. Here is a summary of what he has to say:
Certainly every Companion did not have the opportunity to be in constant company of the Prophet (s) to record his pronouncements with a bearing on the ahkam. Rather, during his lifetime only some of the Companions were present at a time. Thus when some of them heard the Prophet's answer to any question, others missed the opportunity. When the ashab scattered far and wide after his (s) demise, so also did the ahkam narrated by them from him (s). Thus only a part of these ahkam were narrated in any town, so that the ahkam that were accessible to someone living in Madinah differed from those accessible to someone residing in Egypt. Likewise, that which an Egyptian knew was not known to the Syrian; what the Syrian had received was not accessible to the resident of Basrah; that which was accessible to the Basran was unknown to the Kufan, and so on. The result was that each of them resorted to ijtihad on issues regarding which traditions were not available to them.
Further, due to the absence of uniformity in knowledgeability, grasp, and other powers and faculties, these mujtahids naturally differed in their ijtihad and opinions. Therefore, mere individual differences among the sahabah resulted in differences of fattva, and later this difference grew after the period of the sahabah.