It was part of the compromises forming the Constitution. SOME powers would be left to the states; some would be federal. It's one dimension of those "states rights" conservatives love to evoke when attacking any Federal infringement on local reactionary policies, e.g., the racist effort to reduce voting by minorities, or the imposition of Christian ideology into schools (despite the Constitutional separation of state & religion). That said, given local powers to set educational policy, you should expect those policies to vary from state to state depending on the local balance of political power.

Although we have an interstate compact in the US, the public continues to value control of education at the state level.

Article 10 of the constitution reads as follows: reserves powers not delegated to the federal government, nor prohibited to the states, to the states or the people. This is typically cited whenever advocates of national standards on public K-12 are proposed. On a more practical level - the lion's share of funding for K-12 public education is generated from state/local taxes. This limits the influence the federal government has via power of the purse.