There are several reasons, of course. Scandal fatigue, and the complexity of Iran-Contra versus Watergate come to mind as two major ones.

But what I think is the biggest difference is when you compare how Nixon handled John Dean to how Reagan handled Ollie North. Nixon was basically setting up White House Counsel John Dean to be the fall guy, and trying to fool him into it. Instead, realizing that he was being set up, Dean chose to cooperate with prosecutors while staying on as White House Counsel helping with coverup. Finally, he proved one of the more charismatic people in the testimonials and dropped the bomb that Nixon recorded his meetings, which began the death knell.

Reagan correctly identified Ollie North as the key liability. So, when Reagan had his press conference detailing the affair and North's termination, he immediately called North thereafter, calling him an American hero, and apologizing. Although there are no records of an explicit quid pro quo, it seemed fairly clear that North felt protected, and, when summoned to Congress, came out guns blazing. And, in the end, North wound up free of criminal liability, and unironically ran for the senate.

There are loads and loads of other factors, obviously, but this was one of the key things that was done differently - keep the people who can sink you close, rather than try and offer a sacrifice.