Why did slavery become legal?

The Constitution of 1776 protected slavery.

The big debate was the right of individual states to permit or forbid slavery.

Slavery became legal because the leader of the country believed that if he had slaves his life would be easier and he thought this because everybody thought that it would help them out.

NEW ANSWER:

It wasn't the Constitution, but the Declaration of Independence in 1776, that dealt with the issue of slavery.

Thomas Jefferson, who wrote the document was a slave owner who lived in Virginia. However, he wrote a section in the original Declaration of Independence, that would have ended slavery forever. Jefferson argued, that if the United States was going to fight for freedom and independence. Then, it should be for all people, and not just a few.

The other representatives from the South were furious! They refused the passage of the document, unless that statement was entirely removed from the document. Since all of the 13 Colonies had to vote "Yes" in the passage of the Declaration of Independence by a unanimous vote, Jefferson was forced to remove the statement.