Sandstorms aren't technically a "weather" phenomenon (speaking of the real world). Castform changes form due to the forecast, hence the name of its ability. A sandstorm would technically be a natural disaster, much like thunder or a hurricane, which are other moves in Pokemon which relate to weather. That being said, the weather associated with each of Castform's forms are phenomena that originate in the sky, i.e. sun, rain, and hail, while sandstorms and fog are phenomena that are more local to the terrain. It would make sense then that Castform only changes form for those three types of weather.