That's because they have the same skeletons in the closet and are harboring secrets, that if allowed to leak out and be prosecuted would bring down both of them in disgrace.

That has to do with the CIA and Bush Sr. running drugs for Iran Contra out of the Mena, Arkansas airport while Clinton was governor and the septum in his nose was dissolving (that's why his medical records were never made public). The money was being laundered by a company run by Vince Foster, hence the reason "he shot himself", and the records slowly disappeared from view. That's why ALL the records for Bush Sr. and the younger Bush as Governor are locked and secreted away for all time. They knew investigators could get nosey and they wanted to cover their tracks.