Slave trade has brought other countries to become poor for several reasons. One is being that that as their only income. Another reason is wars. Since some countries who let Slavery stay, has other countries gotten mad so they declared war bringing the other country even more in debt due to less people, armory, and etc.
Others are becoming richer by having more allies supplying, making farms instead of slaves doing so, more population to make even more money, and less threat to society.
(Now, I wrote this when I was in 7th grade. Therefore some schools may have different opinions on the answer. You might not get it fully right, but it's something if nobody comment back).