[nippoo]

There's a couple of factors at play here. One is that AC suffers from capacitive losses over long distances (high power multi-megawatt underground/undersea cables are often HVDC for this and other reasons).

The other more interesting one is that the repeaters in this kind of fibre optic cable are usually powered from both ends, from completely separate electrical grids (so one side sends -5000V and the other sends +5000V, for example). This allows for some level of redundancy as well as thinner insulation. With AC, keeping the phases on both sides aligned would be impractical, as well as the inherent inefficiencies of AC transmission.