Maybe the DOJ has looked at the role the dossier played in the Russiagate investigation and concluded that there's no conflict of interest in how the FBI used it, or whether it used it at all. If so, the Clintons could be investigated through normal Justice Department
channels, although ironically that might be more dubious politically for Trump and Sessions than a special counsel would be. Handing off the Clinton matter to a respected outside lawyer a la Mueller would take the administration's fingerprints off the matter. Having Trump's own deputies at the DOJ handle it means that any adverse finding about Hillary will be dismissed by the left as Trump running a banana-republic Justice Department
that's carrying out his political vendettas. If they're going to investigate the Clintons, a special counsel is better for Trump and Sessions than the alternative.