Because that’s where the defense and intelligence industry is centered.

This. Along with the federal government more generally. And also all of the companies that lobby or otherwise work with the federal government. NoVA is adjacent to DC and has a better tax environment than MD, so that's where all the government data centers are going to go.

Also the first transatlantic cables terminated there. Once that started, a lot of cross country fiber wanted to locate there too. Once ghat starts, a lot of other fiber wants to interconnect with existing cables.

Once critical mass was achieved it became much easier to build data centers there. Everyone knows how to do it. Power companies are used to requests for redundant power. There’s talent to build and maintain them…

cheap land (before), cheap and lots of electricity Property tax/sales tax/use tax incentives High fiber density/“network” effect (literally haha)

Somewhat central to most of U.S population

AOL and the early ISPs set up base there, and the first IX was created there.

Now if you want to go back further and ask why ISPs set up shop there, that's likely due to location and nearby population.

MAE-East[1] was there, which means all the networks are there, which means it's a great place to put a datacenter. If you only have one datacenter on the planet, it should probably be in the US; the US has good connectivity to the rest of the world, and local customers with lots of disposable income. If you have one datacenter in the US, DC/Northern VA is a good place, near to the bulk of the population. If you have two datacenters, one on the west coast is a good idea. If you have three, Chicago or Dallas.

[1] https://en.m.wikipedia.org/wiki/MAE-East