Because all Norwegian mariners have the same education and certificates and can go work on the FOC vessels when they have sailed coastal or in the Offshore fleet. The system is not ideal and the government is trying their damn hardest to ruin it. But if we had followed the original thought behind NIS, we would have a international trading fleet of vessels with Norwegians on the top recruited from NOR vessels.
If all the American owned vessels sailing under different FOC could sail in a second US register that would open up hundreds (maybe even thousands?) of spots for US Masters and Chief Engineers, and you would create upward mobility from the US coastal fleet.