Earlier quoted context omitted.
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.
The same argument of being harmful to competition could be extended to schools and libraries and . It's just mind blowing why we should care about companies more than people. So what if it's harmful to competition? What merit does that have? (These aren't questions for you. They're rhetorical.)
This doesn't work out though in rural areas where it's unprofitable for a business to open a branch or provide service. This would be the case for postal service in large parts of the country just like it's the case for Internet providers.