Derailed threads are a part of all forums. It's just the natural flow of conversation. And while yes, it can be frustrating at times, if you step back and look at how communication goes between people, that is exactly how it happens. Rarely do two people stay on one subject for long, let alone a group of people.
Also this site has been around for over 15 years. My understanding is they have tried different levels of control over what goes on in the forums, more strict and less strict. This is the balance they have struck. Now, it's easy for me, or most people, to criticize that decision, but it's also true most of us have not ran or moderated larger chat sites and forums.
For example, i once visited another Christian chat site and jokingly used the term 'good lawd'. A member complained to me about it being blasphemy. When i defended myself a moderator stepped in and said that if i used it again i would be banned. I left a few minutes later and never went back. In comparison to other sites that have little or no moderation and have more trolls than members.
Thrown in that all mods/admins are unpaid volunteers that use their personal time on the site to also enforce things going on, it doesn't leave a lot of room for much stricter enforcement. As well as the majority of reports against other members go ignored because the reasons for the reports are almost always personal and have nothing to do with the posted rules. Perhaps if people weren't so eager to report over personal issues it would leave more time for mods to deal with more genuine issues.
While i think things should be tightened up in that area as well, i also know i don't have the experience of running a site like this, so i don't know all the difficulties involved, so i limit my complaints.