As I said before the Niners were a team built (and built well) for the passing game. A team like this recruits good QB’s (Young was the best second string QB in the league for years, then came out of Montana’s shadow to become one of the best QB’s in the league) and then drives up their numbers.
And winning Championships doesn’t hurt either.
When your defense is keeping the other team off the board you don’t HAVE to throw is 35-40 times a game. Second of all, Marino didn’t get sacked. He was as mobile as a statue but damn if he didn’t have a fast release. And, he had some great offensive lineman in front of him to go along with very good receivers.