What was the main reason northern states opposed slavery?

It was an unfair business advantage, is the right answer.

Explanation:

The antislavery movement that began in the North during the 1830's is more popularly known as the Abolitionism. According to the Abolitionists, slavery was unfair and thus it should be eliminated. The main reason why the Northern opposed slavery was that it was an economic threat to the economy of the North.

a is the final : )