Critical thinking is not prioritized because it is hard to evaluate.
Due to the demand for teacher accountability, the insane level of competition for college entry, and the political games surrounding education policy, modern public education is entirely centered around examination and evaluation.
Not only is critical thinking challenging to evaluate, but, more importantly, people—read, parents—do not accept evaluations that report bad critical thinking skills. If a child can't answer 2 + 2 or who President Washington was, then they clearly didn't know. But if you ask a question that truly challenges critical thinking skills, and the child receives a bad score, the parents will be marching into an administrator's office with complaints of "trick questions" and "unfair grading". And fear of parent backlash drives American public school administration's decision making.