I am sorry if I am mistaken here since I am not American. But don't you have a public holiday for him (MLK)? How did the government not secure copyright for his

I am sorry if I am mistaken here since I am not American. But don't you have a public holiday for him (MLK)? How did the government not secure copyright for his most famous speech? That is almost like the national anthem copyrighted by a corp. Does at least the president keep copyright of his speeches?