EDIT: It's not even that, people are just straight up killing these poor orangutans.
> “Worryingly, however, the largest number of orangutans were lost from areas that remained forested during the study period. This implies a large role of killing.”
> In February, Indonesian police arrested two rubber plantation workers in Borneo, accusing them of shooting an orangutan multiple times, decapitating it and then throwing its body into a river. The men claimed they were acting in self-defense, according to local media reports.
> So, in addition to protection of forests, we need to focus on addressing the underlying causes of orangutan killing. The latter requires public awareness and education, more effective law enforcement, and also more studies as to why people kill orangutans in the first place,” he said.
Source: OP