That is because, like most politicians and bureaucrats, they become entrenched in their own little fiefdoms, and lose sight of their purpose, which is to create and maintain an excellent educational environment, and instead only act in a way that enhances their own power base.
If we approach this using a business model, the school board corresponds to a board of directors, school administrators to executives, teachers to those who produce the product, and parents as the consumers, we can see how to prioritize things. Teachers know more about how to improve the product because they are the ones producing it, and see the results immediately, so they should have more say in what changes need to be made. Administrators should see to it that the business is run efficiently and productively, and keep out of issues that are beyond their expertise. The board should be attentive to the voice of the consumer (the parents) who have the most at stake in the quality of the product, which is the education of their children.
This is not a perfect analogy by any means, but I think it gives us an idea about who should have a greater voice in making educational decisions. Teachers have the most knowledge about what works and what doesn't, and parents have the most at stake in the results.