What happened? I just finished reading "A little Journey Among Anconas." by H. Cecil Sheppard. Joseph recommended the book. Mr Sheppard was world famous for his ANCONAS. They were THE bird in the early 1900's. One would believe that all the commercial egg producers would now be using Anconas. How did the leghorn ever outdo them? Why when anyone wants a backyard flock of egg layers are Ancona not the first bird to be recommended, especially in the cold north? Mr Sheppard's bird lay heavy into the forth year. He stated many of his best producers were four year hens.
They were amazing, but so were the White Leghorns of the day. When the consolidation to corporate agriculture was taking place the White Leghorns were preferred. I believe that they are actually the same thing, just different colors.
However, as to your question about why they are so frequently passed on, the reality is that the culture that supported all of our great breeds and brought them to the fore, went from being the staple to a specialty. We also notice that Sheppard, probably the single best breeder of Anconas to ever have lived, went to the shows in a serious way all the time. By not maintaining many different breeds and by hatching in number--with records--in a professional manner, he was able to select for stock that was both beautiful and productive. Those who continue to breed this way, maintain all sorts of breeds--or at least do their best--that have become neglected by industry and backyard culture.
Industry is easy. Why would they deal with multiple breeds? It divides their efforts. With everyone focusing on production-bred White Leghorns, it focuses all of their collective efforts on making them more and more powerful.
Backyarders have a rather different source of limitation. Firstly, their are more myths and half-truths running around then there are breeds and varieties of chicken. If one says, Anconas (or Minorcas, Hamburgs, etc...) one instantly thinks of "flighty" chickens. People are afraid of that and run for Orpingtons or Rocks. This is a great tragedy. Also, there's the notion that they can't be eaten, which is also a fallacy. Then one goes on line and reads all sorts of misinformation or reads modern poultry literature, which is often replete with misinformation.
Ultimately, there are two kinds of chickens, one of which is divided into two categories. There is corporate chicken, i.e. those breeds that are part of the modern food machine, and there are the APA breeds. This latter category is divided between those who are well-grounded in APA/ABA culture and who work for high ideals and those who are hatchery-based. These latter persist without the necessary information to resurrect, refresh, and revitalize the old breeds because they are removed from the knowledge base and culture that established them in the first place.
Ultimately, many breeds of excellent fowl have been rejected but could very well be reborn if they are embraced by folks who are willing to raise them with the heart of APA/ABA culture.