Why the Error?
A quick look at the certificate details shows that the top level of the path is "DoD Root CA 3". Otherwise, the details of this certificate show these connections are just as secure (in theory) as any commercial certificate. So, the only problem is that the top level of the authority chain is the Department of Defense which is not a "Trusted" Authority.
There are several possible reasons the DoD has chosen to operate as an untrusted authority:
1 - Becoming a trusted Certificate issuer is a VERY hard process. Browser companies have very strict requirements for how a central authority has to be registered and managed to make 100% sure that they are not being compromised. Until recently, self-signed certificates were often used to save money. If you need lots of certs like the DoD does, then you don't want to spend millions of dollars a year just buying certs from trusted authorities. You also don't want to spend millions of dollars on jumping through all the hoops you need to go through to become and retain your status as a Trusted Authority when the only person using these certificate is yourself.
While things like Let's Encrypt and AutoSSL now make this easy to do these things for free, if the DoD already has a solution that works, the time and cost of changing everything over may not be worth it.
2 - In the case of military or intelligence agency certificate authorities, there is also a chance that this is actually a more secure certificate than one issued by a security authority. Basically, to get that Authority status, you need to let a bunch of tech nerds from private outside companies into your systems to verify your methods. If an organization such as the DoD is using classified cyber security methods, then this would be considered a security breach to allow this process to happen.
To clarify: I am not referring to their authentication protocol, I mean that the inner workings of the building and network that houses the server would be classified. Allowing an outside audit would require allowing yourself to be footprinted. Every little piece of information from their techstack, their methods for access control, names of on-site personnel, etc are protected information in a secure facility that you don't just let people have access to lightly.
3 - In the case of a military or intelligence agency you may not trust "Trusted Authorities" because they are such major targets for hackers. If Comodo gets hacked, and the hacker realizes that comodo sells certs to the DoD, then that hacker can start intercepting classified government communications. Or the more likely scenario, China realizes that the DoD is using comodo; so, they spend millions of dollars using every intelligence trick in the book to place a loyal agent as an employee of Comodo so that they use that access to spy on the DoD.
As to the actual question of why browsers do not simply accept them as a trusted authority:
In short, it all boils down to reason #2 above. Browser companies never just give out trust that they have not verified, and the DoD does not just let a bunch of civilians poke around in what is one of their most critical security systems. If the DoD does not let them audit their work, the browser company is not going to make any assumptions just because the DoD should know what they are doing. Infact, a year or two ago Symantec (one of the largest and most reputable cybersecurity companies in the world) failed their audit and nearly lost the right to sell certificates as a Trusted Authority.