There are special rules for certain rare diseases ("orphan diseases") in several major drug regulatory territories. For example, diseases involving fewer than 200,000 patients in the United States, or larger populations in certain circumstances are subject to the Orphan Drug Act.  Because medical research and development of drugs to treat such diseases is financially disadvantageous, companies that do so are rewarded with tax reductions, fee waivers, and market exclusivity on that drug for a limited time (seven years), regardless of whether the drug is protected by patents.
Why are these companies rewarded?
Because medical research and development of drugs to treat such diseases is financially disadvantageous