Well, it may first be worth asking how many Apple Watches have been sold overall. I haven't found any good stats for that, but it seems like it's around 30 mill

Well, it may first be worth asking how many Apple Watches have been sold overall. I haven't found any good stats for that, but it seems like it's around 30 million, and sales may have slowed down since then. Meanwhile the iPhone's sales are in the hundreds of millions, and more are sold per year than there are Apple Watches sold since launch. So it's quite probable many developers don't see the need to make apps for the device. The market isn't large enough compared to other places they could be focusing their resources. Outside of that, as people have said, Apple Watch development was difficult early on, there are fairly few concepts that make sense for apps on a watch compared to on a smartphone, and it's more challenging working within the constaints of a watch than a phone overall. So it seems like more work for a more limited set of concepts on a device that may not have a market large enough to justify the effort.