While HVDC might be more efficient (no impedance I guess) there are a lot of infrastructure folks who are not big fans of HVDC. Back in the 80's I was a transmission engineer for Ma Bell and a lot of time and effort went into trying to protect our facilities near high amperage DC installations (like subways.) A fault condition in those can cause significant electrolytic damage to metallic plant. Its inconvenient in telephone plant, painful in water pipes and a serious problem in gas lines. We were constantly checking bonding, placing sacrificial anodes, etc and we weren't the only utility out there doing it.
Of course, over time we put less metal into our infrastructure (plastic pipes, fiber optic cables, etc) but there is still a lot of metallic infrastructure in place that could be adversely affected by DC fault conditions.