We're looking at the wrong map. The map posted here is for concentrating solar technology, systems that use mirrors or lenses to focus the rays of light directly from the sun.
If we are going to talk about flat-plate PV panels, which use the direct light and scattered light as well, we need to consider this map:
http://www.nrel.gov/gis/images/us_pv_annual_may2004.jpg
Concentrating systems work incredibly well in the US southwest (some have called it the Saudi Arabia of solar energy) and less well elsewhere in the country. PV systems' performance is much more geographically uniform.