Why do states choose birds and flowers to represent them?

Most states use them to represent what kind of birds or trees this state mostly has in nature. It also shows out what kind of state this is and what kind of people live there? For Example: Indiana's state bird is a Cardinal now if you wanted to visit there. You would know that most likely you are going to see Cardinals. Indianas state tree A Tulip Tree. Most likely you are going to see atleast one Tulip Tree if you ever visit.