Many sports require specific skills developed by practice and coaching. Many kids are uncoordinated as freshmen in high school as well, and what they can do later you wouldn't predict when you see them at 14 or 15. Ryun as a senior was fast, strong, tall, and coordinated, so he had the physical tools to do a lot of sports, but probably hadn't played much of the other sports, and by that time he was beating world champions, so he had long since ceased trying out for other sports. As a freshman, he wasn't extremely fast yet and he was uncoordinated, not that strong, and probably had no ball skills.