I think it's a combination of all those things, although I do think that at one time they were considered good. When I read the literature from the early 1900's it seems that many farm hens were producing less than 100 eggs per year- perhaps because of nutritional factors, or perhaps the breed- but a hen that laid 150 eggs back then was probably considered an excellent layer. Our modern expectations ( the 300 per year egg layer and 8 week supersize breasted meat bird) are at odds with a true dual purpose fowl too, I think. My expectations are for a 200 egg per year layer from the hen, and a cockerel from that same breed that provides a decent meal in less than 20 weeks. I would be very satisfied with that. I have a dark cornish hen that lays huge eggs for a Cornish (65 gram range), and probably will end up laying at least 150 eggs this cycle- pretty good for a meat bird , but she certainly would not win any shows.
Then it begs the question of what was the cause of the low egg production at that time? With my Javas, they go broody so darn often that I have had problems getting as many eggs for hatching as I wanted. Back then, having a broody hen was a good thing since most folks didn't have incubators. Now days too, if you don't want to have to incubate, then you need broodies. So you're trying to find the balance between egg production and broody hens.