Probably because for most locations, they are actually not very useful, but quite expensive and just cumbersome. They take up space, and create artificial travel-zones, and at the same time they don't even enhance the flow, because most people tend to stand still on them. For safety-reasons (and probably costs) they are usually just around walking speed. So it's overall just a fancy gadget which is only beneficial for heavy luggage or people with special demands. For those situations, we often have better and cheaper solutions.