One very good reason against adopting DC infrastructure: it's difficult to put out. DC fuses are expensive, ever more so as you go up in voltage.
The most common kind is very fast acting: they don't care whether it's AC or DC, they clear in under a half cycle. These are called "semiconductor fuses" because they can be used to protect semiconductors -- diodes and SCRs, the only kinds robust enough to withstand fusing current without severe derating. I see "traction fuses" up to 5000V, but maybe not anything above there, I'm not sure.
You wouldn't want to use these in normal infrastructure, because of that fast blow rating. One lightning hit and the building or neighborhood goes out! It may well be that slow-blow DC fuses aren't so hard to do. I suspect it's possible, but it's not necessarily an easy or convenient thing.
I would think vacuum reclosers would be reasonably useful, still.
General purpose breakers would need to be very different, and much more expensive.
The "duh" answer is, of course, the entire world is AC, so you have to spend a tremendous amount of money developing and producing equipment and infrastructure just to try something that's only marginally better than what's already out there. There's little enough money already to keep existing infrastructure running as it is.
There are also some possible gotchas, like the innate persistence of DC (except when it does go down) makes manufacturers less likely to build adequate hold-up or surge protection into their designs. A big honking electrolytic does wonders for both of these, and is a necessity when you have to deal with a normal 100-times-a-second dropout. But without that, a brief interruption (10s of ms) from a lightning strike or recloser operating, and every freaking thing reboots!
Same thing happens on a smaller scale in automotive equipment. There, restarts are fairly common (brownouts due to cranking voltage), so most equipment is made to handle that gracefully. On the other hand, complete outages are uncommon (it's usually assumed to be battery-backed, which... it is, after all), which is annoying when the battery does need to be disconnected...
I would think it very likely that any technologically advanced alien species would also be using AC. Mechanical systems can be more powerful, certainly -- consider the amount of power flowing through a hydraulic hose, or various parts of a jet turbine! But nothing else compares with the universal nature of electricity, that it can be connected to electric, magnetic, photonic, mechanical, acoustic, chemical and more systems. It's also king on transmission distance. The exact frequency chosen is fairly arbitrary. Alien frequencies would certainly be oddball frequencies to us, due to our arbitrary definition of the second (and likely their equally arbitrary equivalent), but anything from 20Hz to 200Hz should be pretty likely on an Earth-like planet with the same resources we have (even if in different proportions -- we may not have much copper, but we know exactly how to use it). It's very likely they'll use three-phase power: it's a symmetrical system, and electrical power only has two degrees of freedom, making any higher number of wires or phases redundant, and any fewer wires (i.e., two or one) cannot deliver continuous, rotating power. And any alien species will almost certainly develop magneto-mechanical systems (i.e., generators) long before solid state controls are developed, meaning their infrastructure will be built wholly in AC, before DC becomes feasible.
Tim
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