Two things made it go viral.
1. Jason Kottke posted it to his weblog. Either you know about Kottke or you don't. Many journalists do. He's an authentic low-key tastemaker.
2. I shared the map on Flickr with a CC by-SA license. Which meant any publication could republish it for free without even asking me.
I'm still a little embarrassed about it. The map is pretty simple. The visualization is highly misleading and has all sorts of ugly visual artifacts. That Daily Mail article is full of mistakes (including misspelling my name three different ways.) The picture wasn't even really my goal, it was just a debugging workprint off my "real" project, a GitHub repo teaching people how to make maps in Javascript with vector tiles. But the picture looked cool and was easy to understand.
But I'm also proud of the result. It did look cool! And the recognition pleased my vanity. If I wanted I could have landed several years of consulting work off the momentary fame, I had all sorts of requests for custom work based on it. My favorite outcome was the artist Tamsie River took the data, made a giant mold, and poured a hot iron cast of the Mississippi watershed. I'm still sorry I didn't go for the event. https://tamsie.com/River%20of%20Iron.html