Also, heat generated per server was rising until recently.
Since air conditioning cost is proportional to heat generated, it could have well made financial sense in the 80s and 90s to aggressively cool servers. Not nearly as much heat had to be displaced, and the cost of each server was higher.
From then, institutional inertia and infectious repititis (a brilliant term coined by Amory Lovins) kick in, and it just becomes The Way data centers are built.
Until a bunch of cool kids like Google and Facebook show up on the block, who don't care about the old rules quite so much, and they start to question the conventional wisdom.
Energy costs have risen quite a lot. Cooling is a lot more expensive.
Google (see links elsewhere in this thread) have a lot of equipment, and a lot of data about that equipment, and a lot of smart people to turn that data into information.
