I would guess the feds did not go after Trump because he could spend as much of his own money on his campaign as he wished...
Wrong again. Initially, the feds didn't go after Trump because he was still president.
Once he'd left office, Trumpty and his people said he had no idea what Cohen did or why he did it. He said the proof of that was that Cohen took out a line of credit to pay off Stormy. He--Trumpty--claimed he knew nothing about it.
Too bad for him that Cohen seems to have the receipts.