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Uneducated hypothesis: All hominidae species tend to thrive in huge forests, unless they've discovered fire. From the moment a species discovers fire, any individual can unilaterally burn the entire forest (due to negligence/anger/curiosity/whatever), and thus a huge forest is unlikely to serve as a long-term habitat for many individuals of that species.
Counterpoint: people today inhabit forested/jungled areas without burning everything down by accident (as far as I know; it's the kind of fact I would expect to have heard about if true), and even use fire for controlled burns to manage the forest/jungle.