https://www.sltrib.com/sports/jazz/2...men-going-pro/
There was no upside for him to stay at BYU for another year at his age. Unless he merely wanted more college life experience at the expense of pro prospects.
His weaknesses weren't going to improve with another year at BYU. If he's going to succeed he needs to demonstrate performance against a higher level of competition. For example, the G League.