The whole junk mail issue is really interesting.
The vast majority of Americans hate junk mail. You throw it away. It's such a waste of trees, electricity, the carbon footprint, etc. to create and distribute for such a low rate of usage.
Junk mail gets a substantial discount (about 65% off) because it is a steady, reliable source of revenue for the USPS. So everyone hates it and its terrible for the environment but it keeps the USPS going so we not only tolerate it but USPS pricing encourages it.
But like other mail, the USPS's junk mail revenue is also decreasing year over year. Advertisers aren't going to use a method that doesn't result in net gains and as people turn more and more away from the mail, junk mail is seen as less effective. Then, there is also a push from environmentalists and others to end the heavy price discounting for junk mail as a matter of environmental policy . . . but that would likely only result in further discouraging use of junk mail (which is, after all, their ambition), putting additional strain on the USPS financial viability.