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 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 where needed, as well as use imaginary numbers or letters to showcase different values to aid in explanation. Please use concepts from slides. Scientific research and other online articles are optional.