Environmental factors. It comes down to this.
You don’t have to be brilliant, you just have to show up and put in the work to that focus. Some kids commit hard to it and some kids just can’t commit in HS. Distractions, obligations, household, health and other factors contribute to an A, B, C, D student.
There’s no reason a C or D student in HS can’t ever be a successful person while an A student can equally fall from greatness. It’s all possible.