It's a specific long-term strategy called "Starving the beast", implemented by the conservative wing of the republican party. Control of the US Senate makes it easy to persist policies for a long time. It's a somewhat unusual philosophy where you collect less revenue and borrow to cover the difference to encourage fiscal prudence.

The USPS is a target for a variety of reasons. Think of it as a leveraged buyout target... they have a ton of assets, from information assets to real estate. Carving up the real estate alone would make for a ton of patronage. There are also some extreme positions (which have prominence these days in some circles) that the post office and civil service is a manifestation of big, Federal government exerting too much influence over local affairs.

https://en.wikipedia.org/wiki/Starve_the_beast