It seems somewhat random that a celebration starting in Galveston would become the new national day to celebrate abolition. The anniversaries of the emancipation proclamation, 13th Amendment, Appomattox, the end of slavery in the border states, etc would be more likely to become the nationally celebrated day. Why did Juneteenth win out among all possible contenders?
u/jschooltiger had a great post about this exact topic here:
Celebrating June 19th has a long deep history in Texas, and has spread out to other parts of the country as Texans left (sometimes fled) to other states.