From what I gather from the Wikipedia page, it's due to the transmission cables being underwater and hence having a much larger capacitance, which is a larger loss than normal DC loss. (Correct me if I'm wrong on that.)
So, while AC is still more beneficial for transmission via cables in air/ground, DC appears to have less overall loss for underwater transmission.
By virtue of their tight bundling, all composite multi-core cables have high capacitances. However, onshore AC transmission is typically run on overhead lines, with large line spacings and thus capacitance isn't as much of a problem.
DC is always more efficent to transfer.
But it was difficult to efficiently convert to high voltage and back so you only did it when necessary.
The reason underwater links (eg France-UK) were commonly DC is that they linked countries whose grid wasn't synchronised - the same reason long distance grids in the US are DC
So, while AC is still more beneficial for transmission via cables in air/ground, DC appears to have less overall loss for underwater transmission.