The Mil "Group" testing is likely high-touch, high labor
and attended by a lot of per-lot quality documentation.
That adds a portion of the premium.
Another portion is "nuisance premium" where the vendor
doesn't really care if you go away or not, the Mil business
may not be well loved (esp. if vendor has a lot of experience
with that customer base, and has outgrown it) and internally
considered a distraction that they aren't allowed to quit.
Back when I worked for a company with a lot of difficult
Mil products, we'd keep jacking up the price on the ones
that were production pain until the customers went away
on their own volition (telling them "no" would just get
you a phone call and threatening letter from their "Dad",
(spelled DoD) but too high a price, well, that's just business).
Unfortunately for you there's not much call for small-C, kV
automotive grade capacitors.