Republican Party Platform of 1856
https://www.presidency.ucsb.edu/documents/republican-party-platform-1856
Resolved: That the Constitution confers upon Congress sovereign powers over the Territories of the United States for their government; and that in the exercise of this power, it is both the right and the imperative duty of Congress to prohibit in the Territories those twin relics of barbarism—Polygamy, and Slavery.
Republican Party Platform of 1860
https://www.presidency.ucsb.edu/documents/republican-party-platform-1860
That the normal condition of all the territory of the United States is that of freedom: That, as our Republican fathers, when they had abolished slavery in all our national territory, ordained that “no persons should be deprived of life, liberty or property without due process of law,” it becomes our duty, by legislation, whenever such legislation is necessary, to maintain this provision of the Constitution against all attempts to violate it; and we deny the authority of Congress, of a territorial legislature, or of any individuals, to give legal existence to slavery in any territory of the United States.
I know why Polygamy was put into the 1856 platform but not sure why it was taken out of the 1860 platform.
You do realize that “territory” was not a reference to existing states?