As I understand, the US was like most other countries who didn't allow drug advertising. They could only say "ask your doctor about X".
But the big pharma lobby realized that if they advertised drugs to us average shmucks, we'd all march into our doctor's office and ask for these great new drugs; so then big pharma would be infinitely more profitable.
So they lobbied congress, and congress just stipulated that they have to describe all the instructions, warnings and side effects. It's all about making money.