EXCESSIVE LINKS BETWEEN NARRATORS
The reason why the likes if Imam Abu Hanifa are not mentioned in the Hadith books can be seen below. This is a cause of some differences too.

The differences is basically due to the excessive number of links in conveying the hadiths (between the time of the Messenger (PBUH) and the current era). The more the links in conveying the hadiths increased, the more the disparities between them increased. The reason for this is quite clear as well. If you relay a message to someone and there are many links involved in conveying the message, an inconsistency in the message is bound to occur. This is why the hadith scholars have considered a hadith with a smaller number of links as having a preferential factor over other hadiths. Excessive number of links is a major cause for differences, a fact which one can determine logically, traditionally, practically and through sound discernment. According to the Hanafi school, the jurisprudence [fiqh] of Imam Abu Hanifa (may Allah be pleased with him) is awarded preference over the jurisprudence and statements of the other jurists and hadith scholars because, among other reasons, there are relatively fewer links between him and Allah’s Messenger (PBUH). In order to clarify this, the dates of birth and death of the famous Imams are listed below:

Imam......................Birth...........Death...............Age

Abu Hanifa..............80 AH.........150 AH..............70
Malik......................95 AH.........179 AH..............84
Shafi'i...................150 AH.........204 AH..............54
Ahmed bin Hanbal..164 AH.........241 AH...........77
Bukhari.................194 AH.........256 AH..............62
Muslim..................204 AH.........261 AH..............57
Abu Dawud...........202 AH.........275 AH..............73
Tirmidhi.................209 AH.........279 AH.............70
Nasa'i....................214AH.........303 AH..............89
Ibn Maja................209 AH.........273 AH..............64

From this table, it is quite evident that since a period of two hundred years separates Allah’s Messenger (PBUH) from Imam Bukhari and Imam Muslim, there are bound to be many links between them. Contrast this with the era of Imam Malik and Imam Abu Hanifa (may Allah be pleased with them): not even a period of one hundred years separates them from Allah’s Messenger (PBUH).

Among other things, an excessive number of links is one of the causes for differences between narrations. Also, since the compilation of the various hadith books only began in the second century, there were relatively more narrators (at that time). This consequently brought about more differences in the wording of the narrations.

Taken form "The Differences of the Imams"

Yet we hear comments like Imam Abu Hanifa was weak in his narrations, SubhanAllah.