Why are not all of the Tabi'un and those who followed them considered upright (ʿudūl), even though goodness was mentioned in their regard just as it was mentioned in the regard of the Companions?

There is no doubt that the Tabi'un were superior to those who came after them. However, their era witnessed tribulations, the emergence of sects and deviant desires, and the beginning of the fabrication of Hadith. Therefore, it became necessary to verify those from whom Hadith were taken. Scholars were then commissioned to investigate this, as Ibn Sirin said: "They did not used to ask about the isnad (chain of narration). But when the tribulation occurred, they said: 'Name your men for us.' So, the people of Sunnah would be looked at and their Hadith would be accepted, and the people of innovation would be looked at and their Hadith would not be accepted."

With the beginning of the second century, and with the appearance of narrators who narrated Mursal (disconnected) and Munqati' (broken) Hadith, and those whose errors were numerous, and this increased in the era of the younger Tabi'un after the year 150 AH, and the emergence of sects and the influx of foreign cultures, and the appearance of those who intentionally lied, the scholars of Al-Jarh wa Al-Ta'dil (discrediting and accrediting narrators) were compelled to expand their criticism of narrators and isnads. Thus, scholars like Shu'bah, Malik, and Ibn Al-Mubarak emerged, then Yahya ibn Sa'id and his students, and after them Ahmad ibn Hanbal, Al-Bukhari, Muslim, and others. This continued until the end of the era of narration in the last part of the third century.