Yes, it is only Americans who celebrate Juneteenth because they consider it as a national holiday. It should be remembered that this was the time when slavery was removed in the United States although it took even longer for all of the slaves to be completely freed. Even up to now, it is possible that there are still some slaves. The Americans celebrate this in 45 out of their 50 states.
There are still five states that do not consider this as a national holiday although a lot of people are working on it so that all of the 50 states will be united in supporting what this event stands for which is freedom. When you think about it, who does not want to be free?