SOX may have a special place in lighting enthusiasts hearts but hard economics rule the real world...I would suspect that back in the real world of the 1930s, it was the same hard economics which provided the impetus to develop the low pressure sodium lamp in the first place. The ability to distinguish colours at night was not considered important back then.
The same hard headed economics subsequently allowed it to become the municipal lightsource of choice in the 70s. The fact it provided the highest lumens / Watt continued to make economic sense, especially during the OPEC energy crisis at the time.
Had the same economics continued to be the primary objective back in the 80s, the SON lamp would have unlikely made any major headway in streetlighting. The dominance of SON had less to do with economics and more to do with fashion and an increasingly unsustainable trend to blast the streets with more and more light. Whilst SON does have a longer service life than SOX, surely any savings are reduced due to the increased amount of columns, lanterns and mounting heights required to provide this excess illumination.
The technology used to light our towns and cities only changes to something different, if there is a valid (and profitable) reason to do so. In the early 1970s we changed to SOX because (amongst other reasons) the UK was running out of affordable fossil fuels to burn in her power stations.
In 2016, we are now changing to LED because the UK is running out of affordable power stations.