My guess is (1) because it’s low-hanging fruit, and/or (2) the AG thinks this is bigger than it looks and wants to put him in a position where he might be more willing to cooperative.
It seems like a silly bureaucratic thing—and pretty minor in the grand scheme of things, especially by GOP standards—but I’ll admit I’ve gotten increasingly hardcore about limiting money in politics. I’ve spent a lot of the past four years wondering how it’s possible for a (supposedly) free society to arrive at a point where a handful of powerful people can flagrantly ignore the best interests of everyone else, and I think the core mechanism relies on using wealth to build dynasties. You do well in your private business, you get elected, you help your friends and kids get elected, and it grows.
I absolutely feel for his family, though. Maybe he’s not even wrong that this is a political hit job.