My theory is that the fact that they were created so near to the advent of the current commercial meat birds, that they didn't gain enough popularity before they were replaced by commercial chickens. Also since they were developed brown eggs have become the egg that demands a premium rather then the white eggs like when they were developed, and there are plenty of dual purpose brown egg layers.
I always hear people lamenting the fact that Lamonas fell by the wayside [although the originator did leave the recipe] but I've always wondered why thet became unpopular if they were, in fact, so desireable?