“It did free the slaves—but not all of them,” one student volunteers. It’s a nuanced point, which Klein confirms. “The Emancipation Proclamation freed slaves only in those states under Confederate control,” he says. “It didn’t affect slavery in the North.”
The reason is that, contrary to popular belief, Lincoln’s primary motivation wasn’t to free the slaves but to win the Civil War and reunite the Republic.
https://www.bmcc.cuny.edu/news/did-lincoln-really-free-the-slaves/
Historic ultimatum
“The proclamation gave the South 100 days to end their rebellion,” Klein explains to the class. “If the South complied, slavery could continue. If they kept fighting, their slaves would be freed and could be drafted into the Union army.”
https://www.bmcc.cuny.edu/news/did-lincoln-really-free-the-slaves/
"Why didn't Lincoln free all the slaves in the 1863 Emancipation Proclamation? Was there still a risk of border states seceding?"
"Because it wasn’t politically, strategically, personally or practically possible. In fact, the Emancipation Proclamation freed no slaves at all, as in September of 1862 no seceded state was yet under Union control."
https://www.enotes.com/topics/emancipation-proclamation/questions/why-did-lincoln-not-free-all-slaves-emancipation-292142