Because transporting information is a hell of a lot cheaper than transporting electricity.

The only product Google sells is digital information. Transporting data is dirt cheap. So Google could care less where the data is, as long as they can access it quickly.

Transporting electricity requires big cables made of very expensive metals. Power transmission systems are massive and require a lot of maintenance. They are affected by wind, ice, and lightening. The amount of power Google uses is not at all trivial to have run into urban or suburban areas. Worse yet, when electricity is transmitted, a lot of it leaks out along the way.

Compared to electricity, transporting information is dirt cheap. Data can be transported by much less expensive and much smaller fiber optic cables. Fiber optics require a lot less maintenance than power lines. Lightening strikes, ice, and high winds don't usually have any impact on fiber backbones. Better still, comparatively tiny amounts of electricity are needed to maintain data integrity over long distances. And unlike power transmission, the valuable stuff being transmitted doesn't leak out along the way.

All Google cares about is getting the information back and forth between its users. So it really doesn't matter where the data center is. Electricity is even cheaper at places like Canada's James Bay project. I suspect the only reason Google doesn't go to places like that is the difficulty in getting quality staff to work so far north and so far from "civilization".