Threads are topic-oriented, therefore members who post things that take the thread off-topic are doing a disservice (and possibly disrepecting) the original thread poster. Threads cost nothing to start (they're free!), so there's absolutely no reason not to start your own if you feel like you want to get into something not directly related to the original topic.

Threads are closed most often because one or more of the members participating in the thread have crossed over the line and violated one of the community guidelines. An explanation is posted to the thread about the community guideline violated.

That is not to say that the topic is always off-limits. The topic itself may have been fine, it's just that people's discussion of it went in a direction that is prohibited by the community guidelines. It varies from situation to situation and every thread and situation is judged on its own merits.

DocJohn