One reason is that big pharma has so much influence at the FDA and in Congress. They spend more on lobbying than anyone else. Also there is the revolving door. They hire former FDA officials and former congressman to lobby for them.
A glaring example was when Billy Tauzin chairman of energy and commerce (has jurisdiction over big pharma) retired he was hired by big pharmas biggest lobbying group as the head of it. There are laws that say a congressman has to wait so much time to become a lobbyist but they just get around it by giving him another title. I think this happened right after medicare part D prescription drug coverage was passed. That was a big boon to big pharma. the guy who gets it passed retires and goes to work for them for big bucks.
Big pharma does there own studies and the FDA approves or denies them. If you have the FDA in your back pocket you are in like flynn.
They have cracked down on the marketing in recent years and made it illegal for sales reps to give gifts to doctors and there staffs. My brother in law worked for phizer and was always setting up free lunches and free gifts, pens, ect. The salesmen tactics they use do have a big influence on doctors. Some doctors refuse to even meet with them.