[smarinov]

I think the only criterion is the presence of a person who was able to create an interesting and suitable problem for the target language and submitted it for inclusion in the IOL. :)

Also keep in mind that this is a link only to the sample problems. The actual problems by year had at least Old Indic and Sanskrit poetry: https://ioling.org/problems/by_year

Maybe also others I couldn't see. Also most probably (some/many? of) the national olympiads in linguistics for the participating countries in the IOL had some problems for Indo-Aryan languages at some level (local, regional, national) over the years. On this website we only see the problems for the international competition. I expect that at some point there will be more languages from more language families.