Being a non-seafaring person, I can only guess based on my limited geographical knowledge.
- Longer route. Bypassing Straits of Malacca means traversing on the outer side of Indonesia. Current route into Straits of Malacca hugs Malaysia’s coastline.
- Stronger currents. Outer side of the Sumatra island have to face the stronger currents generated by the Indian Ocean.
- Shallow and irregular waterway at Straits of Sunda. Without sufficient clearance, tankers and cargo ships would not be able to use the straits.
- Infrastructure. There is no sufficient infrastructure on the outer side of the Sumatra island available for cargo ships and tankers to use. The Indonesia government has to invest heavily on creating the necessary infrastructure from scratch.