There are at least three ways to answer this question.
First, we can simply say that the European imperialists were drawn to the natural resources of Africa because those natural resources were and are so abundant. There were so many kinds of mineral wealth in Africa even before it was known that things like oil and uranium could be useful. It is natural that the imperialists would be drawn to these resources.
Second, we can say that the imperialists needed these resources because of the rapid industrialization that had been happening in Europe. Before the Industrial Revolution, these resources would have been less important. With industrialization, though, the resources became much more vital as European economies used raw materials in vast quantities.
Finally, the imperialists were attracted to these resources because Africa was the last continent that remained relatively unexploited. There was really nowhere else to turn for new sources of minerals. Thus, European imperialists were drawn to Africa because natural resources need for industrial production were there in abundance and were not yet claimed by any European country.