Maybe because the new crowns are only available through the extracurricular to spend ECs. Since you’re not able to farm in game for EC items, I assume it’s something to that effect as well? So that players can’t farm others for the crowns.
Also, the black crowns were for a limited time (just BF hunt), while these cash bills seem to be permanent. The black crowns were probably thus tradable so that people could collect them more easily within a short time period, and ATA probably assumes that within an unlimited time period, people that are interested in purchasing the avis with the bills will have all the time they need to collect the required crowns.