The vast majority of the routes that the USPS runs are money losers. Try delivering a 44-cent piece on a mule to the Havasupai at the bottom of the Grand Canyon.
Don't you think, the way the Congress can be bought, that a private company that thought it could do the USPS' job wouldn't have bought its way in?
The vast majority of mail isn't delivered to the money losing routes the USPS always likes to mention (mules, snowmobiles, bush planes, etc). The vast majority of mail is bulk rate and sent from/to urban areas.
Don't you think, the way the Congress can be bought, that a private company that thought it could do the USPS' job wouldn't have bought its way in?