The amount of narcissism these people (the Koch's, for example) have is just astounding. If you're a billionaire, what could possibly be a reasonable explanation for why you even care about the post office? Why are these people so hell bent on hurting people to feed their own narcissism? The USPS could be improved but struggles primarily due to active efforts to kill it off. But it's a wonderful idea of a service tha…
It's ideological. They believe that the government should not be in the postal business. They probably believe that it's harmful to competition in various ways, and they are probably right in certain narrow contexts.
If you make money this way, untapped resources don't necessarily look like untapped oil fields, instead they might look like entire industries that (with friendly legislators) can suddenly be opened up private profit (especially yours), or industries that can be captured into monopolies, or nations whose current economic situation can be exploited in ways that put large amounts of money into your pocket.