Malawi is poor because there is no reason for it to grow above the normal growth rate. Countries usually find themselves in a high growth situation if they are at the cross-road of major trade or geopolitics (ie: Singapore, UAE, korea, Taiwan, etc.) Of course the opportunity has to be cultivated but it has to be there in the first place.

There isn't much in Africa especially in the part of Malawi. They are not even coastal (they are landlocked) which makes their situation even worse.

Botswana is also landlocked, and not very fertile. They have diamond mines though.