https://news.mongabay.com/2017/12/dna-analysis-shows-sumatra...
While humans have no doubt accelerated their demise, the overall problem stems from "rising sea levels that submerged the land bridges connecting the islands of Borneo, Java, and Sumatra to the Malay Peninsula and mainland Asia, subsequently fragmenting suitable habitats for the rhino."
Finally, Sumatran rhino populations are so small, remote, and fragmented that poaching is less of a concern for this species than issues like land use.