Demand, Avgas, Terrain, Cost.
No demand, Avgas not readily available, poor terrain including the rivers (logging throughout region - floating FOD) and the cost effectiveness of a few cent boat trip over the dollar value of a one way trip. Tourism is coastal, with river trips being organised locally especially in Sarawak. Sabah tourism also coastal with interior having poor terrain. Generally locals stick to their own areas as this is still a tribal dominated area.
MH do have a commuter service throughout Borneo stopping at nearly every airfield but they don't venture into the Indonesian side (in fact no one does as tourism not recommended there).
This might help, or might not. Depends on your state of mind...
