Copper wires are copper wires. There are 3 of them for 3 phases. They are sized for AC and AC losses.
DC does not require 3 phases. DC does not have any skin effect or corona losses and thus can operate on smaller wires.
The problem, of course, is that HVDC requires a DC terminal which costs money. It is likely quite a bit easier to build two HVDC terminal on land that the electric company likely already owns than rip up and rewire a whole boatload of long distance wiring.
