There are a number of reasons which I'm sure others will go into, but one point to mention is that FedEx and UPS can charge what they like and ship where they l

There are a number of reasons which I'm sure others will go into, but one point to mention is that FedEx and UPS can charge what they like and ship where they like. USPS must deliver to every address and, to my knowledge, isn't allowed to charge more to deliver to some addresses than to others. So FedEx and UPS can make money by doing a large volume of deliveries in cities, and not delivering to rural areas (or charging through the nose to do so). USPS doesn't have the same freedom. In practice, this means that shippers go with UPS or FedEx unless those places don't go to the address, in which case USPS gets saddled with the unprofitable business.