My theory is... There was a fit/temper tantrum sort of situation - "if he doesn't get an HRH he will not get anything!" from the Sussexes, to which the Palace officials said "sure, no problem" and that was it
Unfortunately they seem like the kind of people who wants to get whatever they want, whenever they want it, however they want it. Especially in light of recent informations from sources that before he "cut them off", Charles has given them a reasonable amount of money (in the hundreds thousands mark) to fund their living situation. Which, of course, would never pay for the $14 milion mansion in Hollywood, so they were furious.
While BRF is an international brand, their focus is mostly on UK/Commonwealth countries. I don't see how US history is important to them while giving the titles, though I do understand people in the US can have an America-centric point of view.
The whole title discussion is basically a non-issue in the UK and European countries with monarchies. The people there understand how it works, so they understand why that decision was made.
And I could argue that this is very much about the feelings - the Sussexes hurt feelings, to be exact. Without it there would be no fuss, no issue, nothing, nada. And pragmatism is having less people with HRHs in the BRF - especially after experiences with Beatrice and Eugenie (and Andrew, who for so long pushed his daughters into the spotlight, fought for their place as working royals, and so on).