Since Ryukyuan languages are genetically related to Japanese, questions about Ryukyuan languages can expose or touch upon interesting aspects of historical Japanese. For example, Okinawan's use of nu as a non-honorific nominative particle is related to the nominative use of の in Classical Japanese. Historical Japanese is explicitly on-topic, so allowing Ryukyuan languages is a straightforward extension.
Ryukyuan languages act as a substratum for the dialects of Japanese spoken in the Ryukyus (e.g. Okinawan Japanese). Hence Ryukyuan languages are directly relevant for discussing Japanese dialects, which are also explicitly on-topic.
Considering the small number of speakers, these languages will never get their own Language and Usage sites. If questions on them go anywhere, it would be here.
For the same reason, questions on Ryukyuan languages are unlikely to swamp pure-Japanese questions or detract from the JL&U site.
JL&U has many users in Japan, who are more likely to be familiar with or interested in Ryukyuan languages than users on a general Languages site.