In my opinion, in this area at least it started years ago with pinkeye/cancer eye issues and a perception that they were in general a weaker animal thus more prone to a range of health issues in general.
Then as the continental breeds became very much in demand they took away from the smaller framed British breeds. Then as the continentals began to fall out favor for issues of their own rite the Angus machine came to prominence with moderation and carcass quality being touted and thus the black hided era began, leaving Herefords behind yet again.
As a Hereford enthusiast I will say that as of right now I think the main reason they are overlooked is because of the market dock stemming from all of the above.
I am of the belief that Herefords can add a lot of positives to a herd. My experience has been that as a whole they are more docile and easy to work with than others. In our herd they seem to calve more regularly and raise heavier calves than our black cows.
They seem to me to crossbreed well with other breeds, the BWF is a good cross for about any scenario.
One of the thickest calves ever raised here was a Hereford x Limousin.
Years ago I crossed a Hereford bull on some Holstien and Jersey cows. Angus worked good on them, but I thought the Hereford added some more volume and size to their calves, and the resulting heifer calves made some fine cows.
We currently have a Hereford bull with cows and another young one soon to go out.