If electrical test based on the
Nat. Elect. Code than Why not 1
License for the entire US ?

That's a good question, my best guess is that because each locality within each individual state can and often does decide for themselves. Having everyone agree on the same thing would be a tremendous undertaking.

Some states, like New York, have no state licensing. It is up to the individual Counties and townships - sometimes individual towns to determine their licensing requirements. Some areas in the country are still using the 1996 NEC.

Then there's the money issue. National licensing, even State Licensing would mean lost revenues to individual areas. They would have to figure how to recoop that loss without raising taxes.

Another reason might be that each locality may have slightly different views on codes and allow exceptions in certain areas. Allowing them to host their own Licensing exams ensure that you will know any specific exceptions or "extras" they may have.