In the Netherlands we use an opt-out system.
You can get a free sticker for your mailbox that says you don't wish to receive unadressed (junk)mail. We also have a national database where you can enter your details: addres/phone number etc. By law it is prohibited (if you're on such a list) to send you addressed junkmail and/or contact you on your phone for telemarketing purposes.
It all works quite well I might say. I used to get about 34 kg of junk mail a year, now I get nothing

.
The US Congress, in their authority as the USPS's voting stock owner equivalent has not been willing to do that. While we do operate in the red and not in the black revenue from stamps is used to fund more then just universal access to postal service for the nation. The goose that lays the golden eggs for that service is the bulk advertiser. The logistics envolved means that although they pay a lower price we still make a profit.
A regular 1st class letter is picked up from mail boxes all over the nation and flown, or trucked locally without delay with too many layers of management looking over everyones shoulder to make sure it is not delayed. I make the example of the beginning of Tom Hanks Castaway when as a FedEx manager he sends the truck, a postal supervisor has the opposite prejudice he holds the truck to make sure the last letter gets on board.
Bulk advertising on the otherhand is not subject to such money wasting handeling. In many cases the advertiser, not the postal service delivers it to the carrier station in route order eliminating the needfor the army of clerks and machines to process it. Not to mention airline and trucking cost. The extra revnue makes it cost effective for me to walk to every mailbox everyday