(Efficient) High voltage AC is ‘easy’ due to how well transformers work, how durable they are, and how simple they are. AC does have issues with inductive loss when buried (or near anything conductive), however. For the same reason Transformers work and are awesome.
High voltage DC is hard as it requires solid state components which are expensive to make, and prone to blowing up (aka relatively fragile). AC to DC and vice versa also adds non-trivial losses.
High voltage DC (to a first approximation) doesn’t suffer from inductive losses however, which makes it much more efficient when near conductive stuff like the ground or seawater.
It’s also ‘simpler’ (doesn’t have things like phase or frequency) which is convenient if doing things like transferring power between two power grids with dissimilar frequencies or phase.
I believe high voltage DC also does not suffer from skin effect. Meaning that the current is transmitted through the whole conductor, not just the surface, leading to less material needed for the same current rating.
The skin effect is frequency-related, with higher frequencies resulting in a thinner "skin". The grid frequency is low enough that it doesn't really play a huge role: a 1000mm2 aluminum conductor has a 10% higher resistance at 60Hz AC compared to DC.
One thing to remember is that it is common for the conductor to have a strengthening core: this allows operation at a higher temperature (which means a higher current), and it allows for a longer distance between support towers (which means lower construction costs). If you're going to put some poorly-conducting steel or carbon fiber in the center of the conductor anyways, why worry about the skin effect?
Besides, those conductors are massive already. At the highest voltage levels, a single set of conductors is carrying multiple gigawatts of power. Even if it were technically viable, would you want to build a single 16GW connection? If it fails, you're going to have serious trouble compensating for that - it's probably wiser to build it as 4x 4GW connections.
HVDC also has its own issues with converter losses and not being able to easily step down to lower voltages. It's very nice for distances in the hundreds of kilometers, but it's just not a viable alternative for short city-to-city links.
Yes, one of the biggest cons (in practice) with DC is the lossy and fragile voltage regulation (both up and down). It is a far simpler form of power delivery though, for better or worse.
AC is amazing for that due to the ease of making/using transformers, as well as efficiency in situations with simple spinning motors, and due to the way it behaves is ‘more safe’ in a number of fault condition.
AC is much easier to manage with fault protect as it has a zero volt crossing point each cycle, and safer for humans from a shock/electrocution hazard perspective due to the reversing voltage/zero crossing point. Muscles won’t ’freeze’ in a given position like with DC, but rather spasm (though it can be so fast it’s basically the same thing), and the zero voltage crossing point tends to allow arc self extinguishing at much lower thresholds.
But the same thing that makes it much easier to voltage regulate (using transformers) is also what creates inductive losses and complicated in some other scenarios (like conversion to DC for non-motor loads, PF issues, etc.).
No one needs a bridge rectifier for a DC to DC power supply, for instance, and DC to DC ripple is dramatically easier to deal with than typical half phase residential AC.
Commercial can have the luxury of 3 phase AC at least, but that is also typically at power levels that it still isn’t going to be cheap building a AC to DC power supply. And god help that power supply if you get phase imbalance.
High voltage DC is hard as it requires solid state components which are expensive to make, and prone to blowing up (aka relatively fragile). AC to DC and vice versa also adds non-trivial losses.
High voltage DC (to a first approximation) doesn’t suffer from inductive losses however, which makes it much more efficient when near conductive stuff like the ground or seawater.
It’s also ‘simpler’ (doesn’t have things like phase or frequency) which is convenient if doing things like transferring power between two power grids with dissimilar frequencies or phase.
They each have their place.