[shiroiuma]

This shouldn't be too surprising. Doing government contracting isn't easy, so certain companies get really good at it, while others simply decline government work because it's too much hassle. That's why there's very little crossover between government contracting companies and other companies: companies that make stuff for the government generally don't make anything else, and companies that are successful in other places don't generally make anything for the government. There are a few exceptions, with the most notable one probably being Boeing (makes both military and civilian aircraft), and also Honeywell.

Honestly, it makes perfect sense fo the USPS to go with a "low-volume turd": they apparently don't want something off-the-shelf, they want something custom for their unique needs. Who's going to build this for them other than a government contractor? You mention the Ford Transit, but the steering wheel is on the wrong side and it probably has some other problems since it isn't specifically designed as a mail truck. You might suggest here that they could contract with Ford to make a custom version, but look above at what I wrote: most companies have no interest in this, and will simply turn down any government contracts, and I imagine Ford is the same way. Why deal with the hassle? For most companies, it's simply not worth it, otherwise they would be actively bidding on government contracts. So a Ford vehicle probably simply isn't an option here, and the USPS's only options are low-volume turds made by defense contractors.