I think part of it also has to do with the amount of patterns they have released in a short span. For a small company like GEC to come out with the Boy's Knife, Indian Girl Canoe, Pemberton, and now the Farmer Lock, that's seems like quite a lot to me. Then there's also the Cotton Sampler, the Eureka, the new Muskrat, and the Northwoods knives they've been working on. I'm sure there are others that I've forgotten about.
They have always done relatively short runs of knives. Some just tend to sell out faster than others. It doesn't surprise me that the Barlows went fast because there haven't been many carbon steel versions available recently aside from the Bokers. I think even those may have been discontinued.