Why did the North wanted to maintain the Union?
Answer made assuming the above question means: Why did the Union not join the confederacy?
The answer is somewhat simple: The union wished to abolish slavery. This caused multiple states start to take sides, and as you can see, the southern states joined the confederacy as they ran large plantations and needed the African slaves to run them. The northern states did not have many slaves due to a lack of need because their food supply/money was made mostly on family run farms.