Why so many data centers in Northern Virginia?

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

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

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.