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.

Maybe because in airports people are sometimes required to walk long distances to go from one point to another, while in a city there are public transport, bicycles, taxis, etc. plenty of other options so walking long distances is usually not required.

They're very unreliable because of the constant wear, slow compared to other modes, and dangerous for crowds and demographics with limited mobility.

Outdoor variants don't protect riders from weather, and having to deal with extremes of cold, damp, and heat makes them even less robust.