On 22 January, 2020, the Ministry of Food and Drug Safety made open useful data: Pharmaceutical products to which patent rights ceased to exist but no generic competition.
The data was retrieved from the “Green List” for linkage patents in Korea (FYI, EPO provides a guideline on how to search the Green List in English, here). From 2013 to 2019, in total 2,629 patents have been registered for 1,501 pharmaceutical products. Of these, 463 products have lost their patents due to patent term expiry or invalidation. However, more than half of them have faced no generic competition (239 products, 52%). These 239 products having no patent and no generic shows interesting distribution: 147 small molecule chemical drugs (61.5%); 85 biological drugs (15.0%); and 7 traditional (herbal) medicine (2.93%). More biological drugs have no generic competition even when patent barriers disappear (Comparing 35.6% of biological drugs in Green List from 2013 to 2018).
The MFDS does not provide any explanation on the cause of no patent and no generic. But I can say that there are three reasons. First, some products still possess regulatory exclusivity such as a data exclusivity. Second, a market size is small and little profitability. Third, for some biological drugs, generic producers were not able to enter the market not because of the patent but because of high cost in producing and complex manufacturing processes.