Before the internet, from the beginning of this country, we had a need to communicate in a cost effective manner.  The government created the post office and kept is relatively inexpensive.  This was part of the infrastructure, along with sewage, water supplies, roads and trains that the government regulated, paid for or subsidized.  Without this infrastructure our economic growth would have been nowhere near as 'great'.

Eventually when our (regulated) phone system (with Faxes) and some healthy competition showed up, the post office was privatized, but this did not mean all the special laws that relate to the post office went away.  Some of these are still very valuable for our infrastructure, but they do place a burden on the post office.

It is easy to forget that not everybody has internet access or can afford the higher rates of UPS or Fed Ex.