Probably started by taking over those massive data centers owned by CompuServe and The Source with their millions of 300 baud modems...
Northern Virginia (DC suburbs) was the original hub of the World Wide Web. A company,Verisign,,was given the rights to assign IP addresses and the url names. Because any email or browser had to go through their translators to get to the right IP, almost all internet traffic in the ‘’90s went through Northern Virginia.
It’s a legacy situation plus favorable tax benefits that so much of the web and cloud is still in NoVA.