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.)
Btw, I don't support privatised education either, and the evidence on various programs where it has been tried (charter schools in the US, academies in the UK, etc.) are fairly mixed. Are they better than the status quo though? Yes. No question. Not even close (the implication often is that teachers are able to just do great work without being incentivized...this doesn't reflect reality).
Also, I live somewhere that had a monopolised last-mile provider. Since privatisation, there has been real improvement in service quality and...amazingly...actual innovation. Is the service good? No. It is still more expensive (somehow) than providers with 1/10th of the infrastructure/network. And the "innovation" is largely copying those other providers. But it is actually getting better (and yes, the main issue is a unionised workforce are holding the company to ransom whilst their market share falls through the floor).