The real question is WHY do the books contain more 'authentic' merits for `Ali?
Fabricated virtues of `Ali:
Why I concentrate on 'authentic' is because the majority of the fabricated merits of `Ali were pretty much all made up quite a while later, in the time of Banu al-`Abbas when the Rafidah finally emerged to the surface. While the Koufans spoke of both correct and false virtues for `Ali, yet these texts wouldn't be circulated as much due to the composition of society at the time, mainly Banu Umayyah who were in full control and other parts of the lands that were run by the Khawarij who also cursed `Ali.
According to some of our scholars at the time, the Shia fabricated more than 300,000 narrations in his virtue and these texts began to spread in the time of banu al-`Abbas. The Qummies continued what the Koufans started at this point.
Authentic virtues of `Ali:
The main reason was that the government policy at the time of Banu Umayyah was aimed at lowering the status of `Ali (for political purposes), their governors would harshly criticize `Ali and verbally abuse him on the pulpits, a lot of greedy people who wanted to win the favor of the government would participate. Secondly, we had the Khawarij who also controlled some of the lands and wouldn't narrate the man's virtues instead they would curse him (for religious reasons).
Anyone who reviews the condition of the scholars of Ahlul-Sunnah at the time realizes that they placed a huge emphasis on propagating `Ali's virtues to counter the government propaganda. There was no real fear for Abu Bakr and `Umar due to the fact that they were loved and revered by the vast majority of Muslims, the government saw them in a positive light and the Khawarij as well. As for `Uthman, there was also no fear for him due to the fact that his family controlled the government and so he was often honored even though the Khawarij cursed him. The Khawarij in principal were god-fearing people, they would never fabricate reports and they saw lying as being equal to Kufr as opposed to Rawafid, therefore they never really did any real damage to `Uthman's character nor would the government allow their ideas to spread.
The virtues of the first three Caliphs:
When the time of bani Umayyah had passed and when banu al-`Abbas came into power the nation witnessed the rise of certain Shia factions, many of them loved Abu Bakr and `Umar although they preferred `Ali, but some of these groups were extremist Rawafid. At that time, the scholars of Ahlul-Sunnah had to propagate whatever popular virtues were known about the first three Caliphs. However, at that point in time the people who had accompanied the Prophet (saw) and accompanied his companions were all dead, so the scholars of Ahlul-Sunnah (since they don't accept lying as a means to promote their beliefs) were only limited to a smaller amount of virtues for Abu Bakr, `Umar and `Uthman.
It is interesting to note that: The rank and status of the first three was preserved based on the fact that their love was Mutawatir among Muslims, the belief that Abu Bakr and `Umar were the greatest Muslims after the Prophet (saw) was mass transmitted and common knowledge to the average layman (until this day) without the need of any texts (Although texts do exist).
Something to think about:
Without checking, I sense that in Shi`ee books al-Husayn has a LOT more virtues than al-Hasan, yet Shi`ee `Aqa'id insist that al-Hasan is greater than al-Husayn.