A "local" scale is specifically calibrated so that some measurable quantity (like the amplitude of seismic waves as measured on a seismometer) gives a somewhat repeatable estimate of earthquake size, but only for a specific area. This is because local scales, like the Richter scale, are effectively a measure of ground shaking. For a given magnitude of earthquake (in the moment magnitude sense, which is a measure of an intrinsic property of the earthquake, i.e., the seismic moment), the details of ground shaking will depend on distance/depth but also details of the rock that the seismic waves passed through between the source and the seismometer. So for the Richter scale and other local magnitude scales, if you try to transport it somewhere else, the magnitude won't be equivalent. I.e., a true Richter magnitude of X in one place won't actually be the same size earthquake of a Richter magnitude of X earthquake somewhere else. That's not a a very useful property for a scale to have.