It seems to eb a matter of age. The rayed craters like Copernicus, Tycho and Kepler are believed to be very young, perhaps only a million years old or less. The material excavated by an impact has to go somewhere and ejection fans are 'it'. Young rocks and rock surfaces have a higher reflectivity than old rocks, and on the moon, micrometeoroid impacts and even particles from the solar wind can significantly age exposed rock changing their reflectivity significantly. The rays are ejecta from craters which have not been weathered very much.