It is because private schools must compete in the free market where the parents are true consumers. Parents who send their children to private schools look at cost and results when making a decision on whether to choose a particular school. If private schools allow costs to soar or results to decline, they know that they will soon lose their customers and cease to exist. Government schools on the other hand do not operate in the realm of the free market. They operate in the realm of politics.
Government schools serve the special interests who have the money and power to lobby at the capitol.
Government schools don't have to worry about parents since they have guaranteed funding through taxes and unless they can afford to leave the parents are virtual captives of the system. The answer is not vouchers. The use of vouchers will merely allow the beast to wrap its claws around non government education and mold private schools into a twin of the beast. Parents, who can, must remove their children from government schools. Enrolling them in private schools or home schooling them.