1 - The volatile political situation in Southern Thailand. In this case Singapore has the advantage due to our political stability.
2 - Singapore as a marine hub is a trusted brand name when it comes to marine expertise. I'm not confusing quality of port Services and the so called short cut route to North Asia. Port of call is still crucial for any vessels on a long voyage journey. I assume the Thais will probably set up a new port to compliment the Kra Canal or maybe improve on the infrastructures of their present ports to assist these major vessels plying the new route. Trust me, the Thai marine authorities are hapless and totally unreliable when it comes to marine offshore and marine safety issues. Why then do ship owners still hire the services of salvage companies based in Singapore each time their vessels run aground or capsized in the waters of SEA? FYI I have been involved in 8 salvage ops. From the Straits of Malacca , to the Andaman Sea, my ex company's Tugboats were in the thick of the action helping to set up containment booms, heavy lifting works and finding bodies of drowned crews.
- Any vessels on the way to Singapore Ports via the Straits of Malacca are not charged any toll fees as it's an international route. The Kra Canal will be a different case. The PRC and the Thais will want to recoup their money for the astronomical amount spent on constructing the canal. For that, ship owners will have to pay for their vessels to go through the new canal. It doesn't make sense for shipowners to pay hefty toll fees for their vessels just to save 3 days of journey when they could save that money by going the usual Straits of Malacca route which I already explained is free from any toll charges.
At the end of the day, the keyword is efficiency and Singapore is no 1 when it comes to that. No ports in SEA could rival Singapore. Even Malaysia tried but failed big time with their port Services.