There is simply no space in most inner cities for this mode of transportation. Which is fine, since inner cities are the most walkable places in the world, and

There is simply no space in most inner cities for this mode of transportation. Which is fine, since inner cities are the most walkable places in the world, and the most well-served by public transit. Unless we're prepared to spend billions on new construction over decades, there is just no infrastructure for a mode of transport that conflicts with all of the existing modes. If anything, we should be building more bike lanes (on a gargantuan scale) since they are proven, efficient and already have substantial infrastructure.