Because he is confused director. He doesn't know whether he is a hard hitting filmmaker or a commercial filmmaker. Result is highly inconsistent and neither here nor there scenario. If done well they won National awards but otherwise they fail critically and commercially.
Prakash Jha started his career with documentaries, made his film debut with Hip Hip Hurray made on Football and was critically accalimed. Then he hard hitting films on caste system and corruption. I still believe that Damul (1984) is his best film till date which not many people have seen but it is one of the grim, stark portrayal of caste politics in Bihar. The film won him both Filmfare as well as National award. His arty films like Damul or Parinati earned him a lot of acclaim but perhaps not much money. So he veered towards working with big stars and big projects. So since 90’s Prakash Jha has been oscillating between socially relevant cinema and commercial cinema. He made Mrityudand with Madhuri Dixit on the corrupt priestly classes (unfortunately a flop commercially), Gangajal and Apharan both with Ajay Devgan, both hard hitting movies and both won National awards and were commercially succesful too. He also made Raajneeti, his most successful film commercially and a very successful web series Aasham.
Now to the flip side. He has been very inconsistent in his content particularly when he is in a commercial setup. He has made middling films like Bandish, Dil Kya Kare, Rahul and even his socially conscious films after Rajneeti like Aarakshan, Chakravyuh, Satyagrah, Jai Gangajal etc didn't had the requisite impact either critically or commercially. Also as a producer only his record is pretty bad. Most of his films as producer are either utter crap (Turning 30, Crazy Cuckad Family, Fraud Saiyan) or underappreciated films (Dil Dosti etc, Ye Sali Zindagi, Lipstick under my Burkha). Not to tell that except Lipstick….which did moderate business all his films purely as producer have flopped. Even his recent web series Sankalp received underwhelming response.
Adding to the woes is his stint as an actor, like in Jai Gangajal where he gave so much footage to himself as an actor that the central core of the film didn't remain on Priyanka and the film suffered due to a weak protagonist. He is a decent enough actor but he cannot pull people in his name as an actor like he tried to do in Jai Gangajal.