A site may be on a very specialized topic, and may open and get some good questions and answers but not enough to satisfy the SE rules for keeping a site open.
For example, on the Podcast #68 blog page it says
We had an Arabic site at time of recording, but it's closed due to low activity levels
I understand why a site may be closed. I see that addressed here: Why close SE sites with low activity?
But I'd like to know if they are ever archived.
It seems to me that for a domain of knowledge that is very specialized and doesn't have lots of contributors, or frequent usage, but has content that is valuable to the people posting it and many readers of the site (even if not to SE), then Usenet is safer, because at least that is fully archived by Google (to the extent that Google can be trusted to keep an archive).