The fabrication of hadith flourished due to the initial ban on the transcription of the hadith that lasted for one century. Had Abu Bakr and Umar allowed the Muslims to write down the hadith, the Bani Umayyah would never have been able to attribute such lies to the Holy Prophet and his companions. Instead, the Bani Umayyah had free reign to encourage fabricators in their favor, and in the words of Abu Jafar al-Iskafi:
Mu’awiyah b. Abu Sufyan encouraged and paid some of the sahabah and the tabieen (the second generation of the sahabah) to fabricate evil hadith against Ali b. Abi Talib in order to slander, defame, vilify, and denounce him. He (Mu’awiyah) allocated an amount of money for them and they started inventing and creating these hadiths for him, which truly satisfied him. Among the fabricators were Abu Huraira, Amr b. al-Aas, al-Mugheerah b. Shu’bah, and Urwah b. Zubayr. 
A prominent Sunni historian adds:
Mu’awiyah ordered all of his governors not to accept any testimony from the Shia of Ali (followers of Ali and the Ahlul Bayt). However, those who were the Shia of Uthman (the followers of Uthman and Bani Umayyah), were brought close, honored, paid, and their stories were documented until the virtues of Uthman were plenty and widespread. Then Mu’awiyah realized that there were too many hadith in favor of Uthman, so he said, “Go and from now on, say hadith in favor of the rest of the sahabah and the rest of the khalifah (Abu Bakr and Umar), and whenever you find a hadith narrated by the Prophet in favor of Ali, invent an identical one in favor of the sahabah. Remove from the public registry (diwan, list of salaries) whoever loves Ali and his family, and do not pay him his salary and destroy his house.” 
As is apparent, the Bani Umayyah were ready and willing to contradict the Qur’an, known hadith, and even exaggerate the qualities and stature of Abu Bakr, Umar, and Uthman for the sake of political gain. Regrettably, they undermined the credibility of the hadith by fabricating a new Islam and turning its history inside out.
 Mahmoud Abu Rayah, Adwa ala al-Sunnah al-Muhammadiyah, p.325
 Ibn Abil Hadid, Sharh Nahjul-Balaghah, 1:358
 Ibn Abd al-Birr, Al-Istiab, 1:65; Ibn Hajar, Al-Isabah, 1:154; Ibn Atheer, Al-Kamil fil-Tarikh, 3:162; Tarikh al-Tabari, 6:77