Why was Maula Ali (as) not able to correct during his rule the changes in Fajr Adhan, divorce rules, and Taraweeh that were introduced by caliph 'Umar?
Imam Ali (AS) has removed the deviations which were created by the previous three rulers of Saqeefah as he clearly declared that they have deliberately changed the Sunnah of the Prophet (SAWA). He removed the discrimination between Arabs againt non Arab Muslims and between Quraish tribe against other tribes etc.
Imam Ali (AS) suffered a lot from the hypocrites and Quraish tribe who were against him and against his reforms. They took the name of Umar as a slogan against Imam Ali (AS). They asked him in the month of Ramadan to appoint for them a person to lead Taraweeh. He clearly said that it is a Bid'ah away from the Sunnah of the Prophet. They insisted and gathered in the Masjid of Kufa. He sent his son Imam Al-Hasan (AS) to stop them from Taraweeh, but they shouted loudly ( WA SUNNATA UMARAAH)(Where is the Sunnah of Umar?). It reached to a critical point of civil disorder among Muslim society. Imam Ali (AS) had no choice but to leave them to do what they want to save the Muslims from turmoil. You can find details in Sunni books like Sharh Al-Nahj by Ibn Abi Al-Hadeed , V. 12, P. 283 and Tareekh Al-Tabari V.4, From page 281 onward.
Shia books like Wasaa'il Al-Shia , V.5 P. 193 has also mentioned some details of this matter.