This is a question that can be answered in many different ways since different people have different opinions about the main causes of poverty.
By some measures, the poorest country in the world is Tanzania. Almost 90% of its people are below the most recognized standard for poverty, which is that they live on less than $1.25 per day. One can look at at least two factors that have caused this.
First, Tanzania was a colony of various European countries until 1964. Colonization is often blamed for poverty. The idea is that a colonizing country does not try to create a strong economy in a colony. Instead, it runs the economy only for the benefit of the "mother country."
Second, Tanzania does not have all that much in the way of resources. It has very little that it can export to other countries. It has a very primitive (in terms of technology) agricultural sector, which makes it harder for it to create surpluses of food.
In the case of Tanzania, then, we can cite colonization and a lack of valuable resources as two factors that cause high rates of poverty.