For me, way back in the early times of 2015, a friend mentioned how much she loved playing Call of Cthulhu in college. A group of us now adults decided we’d try and get a game or two together, and I somehow became the Keeper. In an effort to get myself up to speed on DM’ing (after a looong dry spell) I started listening to Actual Plays, and at the time, the biggest one was RPPR.
At that point, they were adapting Chaosium rules to Delta Green a bit, but there was also bubbling interest in the re-release of Delta Green by Arc Dream. I was focused on CoC content, but I very quickly got into the DG stuff they were playing. I barely missed the original Kickstarter but have made it into all the rest and have a little bookshelf I’m proud of.
How about everyone else?
Someone I’d played with before was like. Hey, I’m planning on running a campaign in Delta Green, who’s interested? And I looked into it and thought. Well, seems cool! And the rest is history.