probably a combination of bot account signups and ancillary bandwidth costs of a costly hosting plan. I've run into and dealt with similar issues in my own political discussion forum and dealt with them. We're about to hit our 10yr anniversary but are likewise on the wain, Facebook / "social media" seems to be vacuuming up everything and simultaneously turning it all into dead ends even while it promises to opposite (and increasingly censors opinions and doubleplusungood wrongthink).
Along the way we found a very affordable unlimited hosting arrangement and ways to deal with the other problems.
I've posted there and PM'd Scott with some ideas, if cost is their chief issue.
If they want to keep it online as an archive I could even host their data in a different directory which their URL could be mapped to. After a very brief transition their site would appear much as it does now for little more than the annual renewal of their URL.