It isn't just weather. Phoenix is probably the closest thing we have to a perfect city for this tech. In addition to the clear and precipitation free weather, its geographic location gives it the most daylight hours of any major city in the country. The roads are almost all big, straight, flat, relatively new, and easy to navigate. There is not much of a pedestrian culture. Drivers are not super aggressive like they are in some other highly congested cities. There is almost no native wildlife that would pose any threat to a car. And I am sure there are other things I am missing.