You have only 20 years with a patent drug before generic brand comes out. Typically in the first 10 years are where trials are conducted and you only have 10 years to profit. That's my extent of knowledge for drug patents
Cost & Time. Why spend the ridiculous cost trying to patent a drug before you have an idea whether it could be successful? If you patent before you start trials you commit to losing future profits on the low low chance that your drugs gets FDA approval. You get 20 years on a patent and it take 7-12 to get a drug to market. Once that patent runs out a lot of drugs lose 50%-90% of profit due to generics. The pharma industry is just bananas.