Why do tree squirrels not occur naturally east of Sulawesi and the Philippines?

The premise that tree squirrels do not occur naturally east of Sulawesi and the Philippines is accurate, as these islands represent the easternmost extent of their natural distribution.1

The reason for this limitation is tied to the Wallace Line, a faunal boundary that marks the transition between the Oriental and Australian biogeographic regions. While the Wallace Line is not a rigid barrier, it represents an abrupt limit for many animal groups, including mammals. The region to the east of this line, which includes the Australian realm, possesses a distinct fauna that evolved in relative isolation from the Asian continental shelf. Consequently, tree squirrels, which are well-represented in the tropical forests of the Oriental region, were unable to naturally disperse beyond these islands into the distinct ecological zones of the Australian faunal region.231