For once, we actually agree on something. There's no evidence that Priest thought any deeper about the DM than "Hmm.. I wonder how I can make T'Challa look as badass as possible in his first panel? I know! Supermodel bodyguards!"
(He pretty much says as much on his website, actually)
And yes, Hudlin definitely raised Wakanda's profile with the whole "unconquered" schtick, which is both a good & a bad thing. Good, because it made the country more unique, but bad because there's nothing some writers like to do more than to break a streak...
I think it's a mistake to assume that the DM were unpopular. It could just as easily been the case that, for one reason or another, the monarchs just stopped doing it.
Maybe there was a past BP who didn't like having them around (because he was happily married, gay, or just preferred to fight his own battles).
Maybe they became more of a ceremonial thing over time, like the Tower Guards in London.
Maybe one of the former leaders of the Hatut Zeraze scored a political point one day and convinced the reigning monarch to disband them.
Hell, maybe they were disbanded the first time a woman took the throne.
It's very easy to ascribe your own motives to fictional creations. The key is to remember that they aren't necessarily the motives the writer had in mind.