The Merkle root is the hash of all hashes. The hash of every single previous leaf node is included in the most recent hash. If you were to remove an old transaction or change a previous block, then none of the subsequent hashes would make sense. The root node has a hash that effectively establishes the validity of all subsequent transactions. If you change the hash of a child node, it will change the hash of its parents which in turn will change the hash of its parent nodes all the way back to the merkle root.