Trump demands loyalty and Pruitt shows that. Pruitt also get Trump’s programs done. Session was a mistake and Trump, his family and America suffers. Time to cure the Session flu.
Also, IIRC, since Pruitt has already been approved by the Senate for his EPA post, he could be laterally moved over to DOJ without having to go through another round of Senate approval (which unlikely would be easily obtained if Trump let Sessions go). If this is indeed the case, another strong reason for Pruitt to replace Sessions.
Get rid of Jeff now, He has been shown to be utterly useless instead of going after crooked hillary and illegals he spend his time and resources on chasing pot smokers