Money is, of course, the obvious answer.
But I think more importantly, it's about demeaning government. There is a widespread notion that government does not do anything well, and private companies do everything better.
The Post Office is an easy target, because people interact with it directly. Every single missing letter/parcel is evidence that the government is bad. (Even if wasn't actually sent via USPS.) Every stamp price increase is proof of government incompetence. And that means the entire government is incompetent.
They may or may not actually care whether the Post Office itself is privatized. But it is an effective ideological football.