Because they'd essentially be giving up some amount of control over the Chrono name if they let that happen. They'd be running the risk of fans not only making a bad game that tarnishes the Chrono name, or coming up with ideas that Square can no longer use without paying these people which I doubt they want to do if they ever do get around to making another game. And whether they ever make a new one or not, they can certainly make a small mint on remakes, and rightfully don't want any random group of people on the internet messing with their top franchises.