Trump had hired him to conduct an investigation of dems etc who tried to prosecute Trump for his crimes. Times reported that DiGenova had convinced Trump he would punish them all and it would be easy blah blah and they now were frustrated he hadn't done it and pressured him to be done before the midterms, so he resigned.
He won't comment on why but it's widely assumed he of course had no actual legal grounds he was just grifting off Trump