Why is Jafari Fiqh, from Imam Jaʿfar al-Sadiq (AS), still questioned and not considered part of Ahlus Sunnah just as Imam Abu Hanifa, Imam Shafi, Imam Ahmad bin Hanbal, and Imam Malik have established their respective schools of Fiqh?
Simply because the tyrant governments which followed the Saqeefa have supported their own sects and opposed the Fiqh of Ahlul Bayt (AS). They admit that their four leaders named in your question came long after Ahlul Bayt (AS) and were never more knowledgeable than Ahlul Bayt (AS). Abu Hanifa of the Hanafi sect was reported to claim that he studied under imam Jafar Al-Sadiq (AS) for two years and was nothing if those two years were not there لولا السنتان لهلك النعمان.
Fiqh of Imam Jafar Al-Sadiq (AS) is the exact Fiqh of the Prophet Muhammad (SAWA) while the Fiqh of others are based in their own opinions and different understandings. Yet, many tyrant governments don’t consider Fiqh of Jafar Al-Sadiq (AS) just because of their political stand against Ahlul Bayt (AS) which they took it from their previous leaders of Saqeefa.
Wassalam.