There are a variety of reasons that a number of data center companies have chosen to locate in Northern Virginia. This area includes Sterling, Manassas, Herndon, and Reston. 1 -- Power is relatively cheap in the area, varying between $0.058 to $0.065 per kWh. It comes from a diverse set of sources, including natural gas, coal, nuclear, and green/renewable sources. The power company is also willing to enter into long-term contracts with large (multi-MW) customers. 2 -- Water is relatively cheap and available, compared to other areas in the US. Water is used for the cooling systems in the local climate, which is more efficient than the Glycol or DX systems used in parts of Texas, for example. Typical cooling efficiency would be closer to 25% overhead in VA, rather than 50-60% overhead using a refrigerant based system. 3 -- Network connectivity is plentiful. As Ben noted, Verizon/MCI is headquartered in Ashburn. MAE East is also located in Ashburn/Reston/Vienna. 4 -- The US Government and related Federal agencies have a lot of demand. Being able to service this locally is a plus to many providers. 5 -- There's low latency connections to Europe through MAE West. This makes it possible to host EU sites from the US, which could have other financial benefits. 6 -- There's a large pool of locally available skilled labor, which reduces the ongoing cost to operate these facilities. 7 -- There's easy access to a major local airport and metro area, which is a factor in getting timely support from hardware and facilities vendors.
There are additional details that I could add if you are interested, but these are the first ones that come to mind.