Is it illegal in West Virginia to eat caviar while driving a Ferrari, reading The Economist, and flossing their teeth? Why isn't that illegal? It's because nobody in WV eats caviar, drives a Ferrari, reads, nor practices dental care. So there is no need to tie up legislation and pass that type of law there.
Likewise, in states like NY, Hawaii, and California, nobody marries their first-cousin. Therefore, it is a waste of time, money, and resources to make things that nobody does illegal.
On the other hand, there is a reason why states like WV, Mississippi, Arkansas, Kentucky, Wyoming, Idaho, the Dakotas, etc. have made it illegal. It's because they had to stop their own people from doing that shit. The fact that you and Crispy Lips have recently Googled where it is legal to marry your cousin speaks volumes.