Its incompetence of the gov not any deal with establishment why PPP got relief.
In case of Nawaz Sharif, there were petitions against him in SC from PTI and other sources. Since Mr Nawaz Sharif was hindering the investigations, refused to step down from the PM post for a unhindered investigation and parliament was not going to impeach him, there was no option but to make him na ahel on very valid violations on his part and request NAB to proceed with the cases. They went light on him and case against him were strong enough to jail him from the start but he is being given every chance so he can't claim wasn't given the chance to defend and become a syasi shaheed.
In case of PPP it was the sitting government's job to file references in SC.
SC did not take unilateral or suo moto action against Nawaz Sharif similarly they will not do it against PPP as well. You need to have a party to file petitions.
NAB cases against PPP members are still open and have not been shut, difference is SC is not actively following up on them since no petitions. Also PPP cases not getting the same media coverage.
Eventually PPP will have to face justice, they are not completely off the hook but to say they are getting favoritism from the establishment is wrong. PPP members have been jailed and they are out only on bails.
Infact the mother of all land corruption cases (Bahria Town Karachi) is proceeding with a media blackout and PPP members will be hit hard from it. SC has voided the land sale as null and void and instructed NAB to find/punish individuals responsible for this fiasco.
SC has barred Bahria from selling any more projects and those people who have bough land from Bahria, in order to protect them, SC ordered Bahria to buy that land again from the gov on market rate.
https://www.dawn.com/news/1405576