Earlier quoted context omitted.
Because a reliable communications network is a public good, and cutting off millions of poor Americans from critical services is a despicable way of remediating whatever vague injustices the USPS is perceived to be committing against taxpayers.
If you want to give welfare to poor Americans give them welfare and let them pay the true price of their goods. I bet most of them would choose to keep the money and have postal service once a week, or every 2 weeks if they could keep the cash savings.
USPS lost $9 billion in 2019[1], so roughly $30 per American. Anyone who pays even one bill per month via mail would pay at least 4x that much at current private rates. I'm not inclined to believe privatization would bring those rates down much for rural areas.
Sure, if we assume we've implemented UBI and eradicated poverty or something then those prices aren't a big deal. But even then, what have we gained by privatizing? Private industries that provides critical services are either massively subsidized (agriculture, transportation - indirectly via roads) or massively unpopular and frequently monopolistic (telecommunications, health insurance). Even if a talented CEO could squeeze a few percentage points of efficiency out of this industry, it would come at the cost of misaligning incentives in perpetuity, and the government would still be responsible for keeping the industry afloat whenever the economy goes bad.
Or we could let them charge a few extra cents for postage, let them operate 5 days/week instead of 6, charge the wealthiest Americans a couple extra tax dollars each, and keep consistent service available at dirt-cheap prices for everyone everywhere.
[1] https://about.usps.com/what/financials/annual-reports/fy2019...