I also feel MBCook is probably right. In this particular case, however, there does not appear to be any argument for prosecution that does not immediately look preposterous as soon as you give it any thought. If this is so, then, by equivocating - for political expediency or any other reason - the DA is inviting doubts about his competence or integrity.
I'm hoping that "the issues at the heart of the investigation have been resolved through non-legal means" is a paraphrase for saying that the DA pointed out to the governor, directly or through his aides, that any attempt to prosecute would fail and bring down a heap of public ridicule.