If states were to ratify the U.N. Convention, many of them would not be able to protect their national identities.  A state-by-state approach would allow each state to pass a law that fits its needs, particularly those of protecting its national identity, which is a concern international law cannot approach. Maintaining an original ethnic and cultural structure is important to many states, especially those that are populated by one ethnic group. Is Israel, for example, wrong to term itself a "Jewish state"? There is nothing inherently wrong with its efforts to maintain this identity, even if that effort constrains the expansion of migrant rights.