[cjbgkagh]

Less noise and dust down there - I think the air is purged with nitrogen as well so no oxidization. They could do the same thing cheaper on land and recycle the waste heat. Edit: just read the article - they also state temperature stability as a factor.

Having the servers more easy to access and repair also makes them more likely to fail. Also spaces that humans can enter require a bunch of regulation that takes up space and adds to the expense. So the math could easily work out in favor of a sealed box of computers that no-one is allowed to access and any broken servers just get switched off.