I also think it has a lot to do with what people already have. Most regulators that have been sold in the US in the past use Yolk fittings. It is not likely that someone is going to replace a regulator just because they are buying a new cylinder, unless they are buying everything together. I also think you are right cost is a huge factor. I also think DIN is used more exclusively in the technical dive community, which we all know is a rather small percentage of all divers.