There are two main reasons for this.
First, there is a political/constitutional reason for this. Our government was set up as a federal system in which states were given powers that the federal government did not have (and vice versa). In such a system, each side will want to protect its powers so the other level does not encroach on them. States will want to limit federal power over their schools because they will not want to lose any of the powers that are given to them by the Constitution. In this sense, it is all about power and protecting one’s “turf.”
Second, there are ideological and philosophical reasons for this. The people of different states have different values and different ideas about what should be taught in schools. People in liberal states, for example, will be more comfortable with the idea of sex education in their schools. They will be more open to textbooks that teach about human evolution and which portray some aspects of American history in a negative light. People in more conservative states will not be comfortable with such things. Both sides will want to limit federal control over schools for fear that the federal government will mandate things (like abstinence-only sex education) that they, the states, feel are inappropriate for their students.