Probably because his opposition, especially the Clintons, know a lot more about what happened a few years back in and around Mena, Arkansas, involving themselves and George I, than his own party members do.
I used to believe/hope that Bush 43 was only protecting his father and his administration out of a wrong but well-meaning sense of loyalty to family even at the cost of harm to the country.
However, when Jeb calls Clinton "bro" and Bush 41 and "Bar" call him "son", a very different and even uglier picture begins to form. IMO it's not just blackmail. The Political Class has more in common with each other than they do with us, even when they pretend to be in opposition.