Interesting what is the issue with 48V, I saw equipment for it seemed to be overpriced. Remember pricing out some stuff and as soon as 48V option for power came into play then price rose quite a bit.
Or is it that voltage is not high enough to be efficient for a large data center?
The computers have extremely simplistic power supplies that basically can't fail, and just DC->DC transform from 48v to 12, 5, and 3.3; and the large scale power supplies that convert three phase 240v (or whatever you're supplying it with) from the datacenter to 48V are much higher efficiency than the ones that would have been in the server (which you would have fed them, usually, something like single phase 208v).
Redundancy is supplied by just hooking multiple transformers in the middle rack to the + and - terminals on each PSU, instead of a convoluted multi-module redundant PSU (which always uses a single backplane, and backplanes in redundant PSUs fail surprisingly frequently).
The total round trip efficiency of this system is about as high as you can realistically get. 80Plus Titanium is 90-95% efficient (depending on load), but has efficiency losses in rack level distribution which 48V tries to correct.
However, 48V DC can be very dangerous to work with, and a lot of tech workers refuse to work with it. Now, if you believe it is dangerous or not (I've seen arguments stating that it is no more dangerous than single phase 208v) is immaterial, this is the opinion of a lot of workers.
The cost of 48V gear is expensive if you're not in a datacenter already setup to handle it. Facebook obviously doesn't have this problem because they build entire datacenters from scratch.
I personally don't believe in it because it doesn't buy me anything that single phase 208v doesn't give me, I do not pay enough in electricity to have the overhead of dealing with it.
While you could get fried from a single rack server's failing power supply, it is likely that it will die of some other cause before zapping you if you take simple precautions.
DC on the other hand, puts "the forces of nature" up close and personal to the back of the rack.
A wrong move will vaporize the thickest of metal screwdrivers and can easily do irreparable damage to a human.
Something as simple as not removing your wedding ring can result in an inadvertent bridge between + and - ; after which, bad things happen.
48VDC isn't sufficient to vaporize a metal screwdriver at the amperages used in most 48V datacenters, however, I know people who work with higher voltages than that, and they own expensive ceramic tools just for safety reasons. If I personally ran 48VDC, and my workers requested ceramic tools, I would not hesitate to expense those.
And yes, if you're working around DC >12v, you should be seriously using every method available to make sure you don't become a conductor, including only using one hand at a time to touch power relays (to avoid having your heart stopped).
Theoretically you should have zero leakage, and you should also trip on bigger currents (but a rack wouldn't use more than 10A maybe?)