Being mildly familiar with the US political system, but none of the specifics here, my best guess is that it was a combination of political donations and hiring

Being mildly familiar with the US political system, but none of the specifics here, my best guess is that it was a combination of political donations and hiring some sort of lobbying company. As a mild form of corruption, it's fairly common for lobbying firms to hire ex officeholders, as well as close friends and relatives of officials. Then the lobbyists have a core of trusted people that can at the very least get the official to listen to their client's argument. There (usually) isn't any direct payoffs involved, but if someone plays by the rules they can generally be assured that there will be a cushy job waiting if they want it once their career in public office is over. So in this case, my best guess is that a firm was hired to have one of the governor's colleagues raise the issue of the pardon every so often until he finally said yes. It also probably didn't hurt that he was the sort of person who can afford to hire their services, and there's always some upcoming election that needs to be financed through campaign donations. But again, this is mostly just speculation, and I know absolutely nothing about his particular case.