How do steel and enforced concrete posts compare?
I thought that there is also a safety angle here. Steel pipes tend to bend and fall over if hit by cars etc, therefore have a better impact for the ones that hit it. Concrete on the other hand does not tend to bend and therefore will cause a lot more damage to those that run into it. Of course this may have nothing to do with what you are seeing.

Here in the UK they are also rolling out collapsible frames that are used for large road signs as well so that if cars hit them they only cause minor damage to the vehicle as they bend and collapse around the car.

edit: The frames are known as Lattix. Lattix Website
      The safety aspects are demonstrated here. Lattix Safety