alt Hacker NewsThat's probably just because there's not much demand for it, not because it's difficult.
There are mines with plenty of space in them, and heat removal can be done with water cooling. But who is going to pay 50% more for that when it isn't needed 99.99% of the time?
Or when the same thing can be accomplished by putting nothing in a mine, but an approach as simple as having geographical diversity by paying for some 'normal' colo in Hillsboro, and more normal colo in northern VA on the other side of the continent, and duplicating your stuff between two facilities. Weird underground datacenters aren't exactly known for having a large number of competing on-net carriers.