Probably because data centers don't necessarily mean a lot of jobs. The 2014 financial report doesn't mention Amazon in their top 10 either: http://www.loudoun.gov/DocumentCenter/View/110442
Looking for headquarters is meaningful if we're trying to make a comparison between Mountain View and Loudoun County. Google is headquartered in Mountain View. Loudoun has no real equivalent for HQs or even large offices for global companies. The startup scene is pretty dry too.
Apart from voting a board that wants to increase housing there's almost no comparison. If you want to use Dulles and Loudoun as an example you should use it to illustrate that you can't slap in a bunch of housing units without thinking about infrastructure.