Because the Postal Services Act 2011 mandates it, as does the EU, and it has been a source of financial difficulty in any case.

The UK is the size of the state of Oregon and about five times as densely populated. The UK also has a much more powerful central regulatory apparatus -- there is no US body that has as much power over private carriers as Ofcom has over Royal Mail.

> Because the Postal Services Act 2011 mandates it

Exactly.

> there is no US body that has as much power over private carriers as Ofcom has over Royal Mail

I thought regulating inter-state commerce and especially mail was something the US federal government has historically done?

In fact isn't there even a federal law enforcement agency specifically for it?

The US equivalent to Ofcom is the FCC, which is not as powerful in statutory authority nor as aggressive in practice. Ofcom, among other things, regulates rates and service days for Royal Mail. I do not believe FedEx or UPS would accept this degree of regulation in exchange for the liberty to deliver letters to mailboxes.

What's also significant is that Royal Mail privatization is less than a decade old. From a policy perspective, it's still an experiment. It doesn't actually have any competition for letter delivery. That Royal Mail has survived despite the lack of a statutory monopoly does not mean that the same would be true of USPS.