i think it's a combination of three things mainly. teacher salaries have not kept up with the cost of living, so the job is financially less attractive every year to capable young people. for those who do take it on, they find themselves swamped with bad policies, low standards of student performance, and a lot of red tape. and lastly, teacher training has come to focus more and more on techniques of delivery and philosophy of teaching and less on mastery of material. it's very frustrating because a general solution in any country takes leadership at high levels, and that has been lacking in the USA.