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Prosecutors in the US are given leeway to make deals under the law that benefit the people. I am not sure of the details, but it often happens that companies indemnify executives (i.e. promise to pay legal expenses incurred defending behavior associated with working at the company). That means that suing the Sackler heirs (especially if unsuccessful) might diminish the funds available to recover from Purdue, since Purdue would be paying the Sackler legal fees. If the prosecutors believe that probability of a successful suit against the Sacklers is low enough, they may do an expected value calculation where they value the private contributions of funds from the Sacklers under the settlement more highly than what they expect to recover in the event of a lawsuit against the Sacklers.