The USPS has been around a lot longer than UPS, so they have always been package shippers, primarily to residential addresses. It wasn't until UPS expanded in the '80s and FedX, RPS, DHL, Airborne et.al. came to make the residential market competitive. FedX bought RPS to create FedX ground. DHL bought Airborne to expand their domestic marketshare.
Keep in mind that the postal service would deliver anything anywhere, mostly losing money in the process. When UPS decided to get into the residential market, all of the big catalogue companies like Sears and Spiegel switched to UPS for the cost savings, making it profitible for UPS. If you asked the postal guys, it's still a money losing proposition from them to be a package shipper.
