A big part of the problem is that teaching is treated as a second tier profession. A lot of people end up teaching after having failed at something else. I took some masters level education classes and these were some of the dumbest people I had ever met in a college environment.
The people that choose teaching as a first choice often leave because the pay and work environment is so poor for such an important job. They either burn out from stress or give up because they cannot properly do their job within the system.