Why Are Pharmaceutical Companies Less Willing to Fund Research on Plants

Why are pharmaceutical companies less willing to fund research on plants for medicinal purposes than on man-made medicines?

A) Because there is not enough land to cultivate the plants in sufficient quantities
B) Because past research has not lead to any useful outcomes
C) Because plants are more difficult to obtain legally
D) Because the plants cannot be patented to allow exclusivity for any profits
E) Because public interest in using herbs and other natural products is waning