Marginal Principle in Pharma Industry

In 2016, the top-selling pharmaceutical drug in the world was AbbVie's Humira., which is used for the treatment of several common, chronic conditions. The majority of its profits are derived from the treatment of the most common diseases, so why does AbbVie develop drugs for rare diseases instead of investing all of its resources toward drugs for common diseases?

Use the marginal principle to briefly explain your answer.

You should use diagrams as well as imaginary numbers or letter to show case different values to aid in explanation.