Take the examples of Dagestan and Moldova. Both have almost 3 million people (including Transnistria for the latter), both were conquered by the empire at roughly the same time (1813 and 1812 respectively), both were conquered from an Islamic empire (Persia and the Ottomans respectively), both have their own ethnic identity and language(s), and in terms of size, Dagestan is larger.
So what determined SSR status / why weren't there more full-fledged SSRs?
I answered a similar question some months ago, but if you have anything else to clarify please let me know.