Message boards are always temporary in nature: content is not meant to be kept forever and can be removed for a number of reasons, including expiration (i.e. when a conversation does not see any activity for a certain period of time). There are some interesting discussions with content worthy of being preserved, but that content is better suited to being stored in permanent parts of IMDb (such as user reviews, FAQs, trivia, goofs to name just a few).
This not only ensures that the information is archived forever, but also puts this content in front of a much larger audience, instead of the smaller subset of message board users, and makes it easily searchable across all of IMDb’s interfaces and services. Board discussions on the other hand are an unmoderated mix of fact, fiction and off-topic banter which is designed to be stored only temporarily, is scattered across a number of different threads and is only really available on our desktop site.