It was the first state to do so. The real question is, why is that such a hard concept that it requires a state law to be passed to uphold it?
Public bathrooms are broken down by anatomical differences — not mental ones. Which bathroom a person uses is not, and has never been, based on what he or she thinks or wishes about him or herself. The bathroom is based on whether or not a person has a penis or a vagina.
The same could be said for most things in society. Getting a job as an airplane pilot is based on whether or not a person possesses the physical capability to fly a plane. People don’t get to walk into airports and fly planes simply because they dressed up in a pilot’s uniform and think they can fly… without actually being able to do it.