The json file allows us to add virtually any number of properties we want, and we can nest them if necessary. We can also leave whole sections of the file blank without affecting the ability to read the file in. CSV gets extremely cumbersome when we start adding dozens of fields for Skills, Personalities, and other fields that we may want to move between college and pro. The json format opens the door for us to allow modding and customization of pretty much any property on teams, coaches, players, etc. so we felt it was a good time to start the process now.
.json files are also text based, so if folks wanted to start open source projects on github related to roster files, import files, history files it would be much easier to update, merge, etc.