Why didnt Lincoln preseve slavery?

Abraham Lincoln was more focused on preserving the union because the south thought that if he won the election it will jeopardize their ability to own slaves. Lincoln had to make the war about the union instead of slavery because he would have more people vote for him, and even thoough he said he wanted slavery to be abbolished that wasnt true. Also he knew that the South wanted slavery, he also knew that the north didnt want slavery so had to do something that both will agree on.