Content erosion and updated (cross-wiki) WikiProjects
Even smaller projects often have one or two editors interested in something. If there is a global wikiproject for that area, it may support them and encourage them in editing on it, on the local project, provide resources to do so, etc.
Separately even if no local editor exists, there is a global project whose members have an interest in the completeness and quality of that topic on all projects, including the small one with no editor. Maybe one time they decide to spend a week and translate their top 30 articles into that language as a "drive".