Ok hang on a second. Crohn's disease is NOT classed as a rare disease when it comes to orphan drugs.
The FDA states that for an orphan drug the following must apply;
"(i) The disease or condition for which the drug is intended affects fewer than 200,000 people in the United States or, if the drug is a vaccine, diagnostic drug, or preventive drug, the persons to whom the drug will be administered in the United States are fewer than 200,000 per year as specified in § 316.21(b), or
(ii) For a drug intended for diseases or conditions affecting 200,000 or more people, or for a vaccine, diagnostic drug, or preventive drug to be administered to 200,000 or more persons per year in the United States, there is no reasonable expectation that costs of research and development of the drug for the indication can be recovered by sales of the drug in the United States as specified in § 316.21(c). (9) A statement as to whether the sponsor submitting the request is the real party in interest of the development and the intended or actual production and sales of the product."
There are more than 200,000 suffers of Crohn's in the USA.
Also a lot of drugs developed for Crohn's are also developed with other indications in mind.