Answer:
It's simply more cheaper. Also, you keep the slaves.
Step-by-step explanation:
Think about it. When it comes to indentured servants, not only do you have to pay for them individually to come here, but you also have to pay more expenses. And remember, indentured servants are buying their freedom. Eventully, you are going to have to let them go. They worked because you hired them, and you owe them that freedom. It's in the contract.
Let's move onto the slave part. Slaves were taken by the hundreds in big boats called slave ships. The living conditions were dim, and often dangerous. Many had died on the way, showing evidence that it was a lot cheaper to import slaves. You did not have to pay for sailing expenses, and even if you did, you payed with a small price. Next, slaves were lifelong workers to you and your family. Because you bought them, they and their descendents had to be working for you, that is until the Emancipation Proclamation, but that's another story.
Hope this helps, do leave a question just in case i missed something.