You can move a much higher volume of cargo by ship, also avoids the need to unload it and move it to rail.
Are they moving an amount beyond the capacity of a railway? And I would expect that the ocean vessels wouldn’t be the same as the ones in the canal.
Edit: From the article:
Shipping containers from those ocean-going vessels would be transferred by cranes at Kep to and from canal barges.
I’m going to go out on a limb and guess they’ve done cost analysis before embarking on this project. I find it hilarious how lay people always assume that engineers embarking on years long projects don’t think of these really basic things up front. Like you really think that you just had this epiphany that nobody involved in the project considered?