What was the main reason john. c. calhoun fought so hard for slavery?

The biggest reason why was simply because he truly believed it in a positive light, added to the fact he was born in South Carolina which was part of the deep south where this was accepted as a way of life. As Senator and Vice President he defended it drastically, especially as a proponent of state's rights and having the individual states decide for themselves whether or not slavery should remain legal 

He believed it provided a "positive good"

Calhoun believe slavery had positive impacts and it was up to states to decide on slavery.

Calhoun fought against the "necessary evil" argument many Southerns had of slavery as he believed slavery provided a good to both sides. Whites benefited from the labor source making the South and the nation profitable. Blacks benefited from the teaching of culture, Christianity, and the provision of shelter, clothing, and protection. Calhoun also fought for states' rights and believed the states had the right to control their economic system. He also fought for the right of nullification meaning states could override a federal law if they disagreed with it.

It has the potential to lead to limits on rights by giving flexibility in making laws.