Easy to learn, hard to master, as simple as opening a browser tab to get it up and running. It was a perfect storm of tight gameplay and low barrier to entry. I also suspect many people hadn’t come across a game like this before- I can’t even name a multiplayer top down shooter like this save for Continuum.
I guess for many of the players of old, it was because of the community, that was comprised mainly of HN folks. So, the chat was very nice and interesting.
Also, a game that's basically 2.5 years old "refusing to die" is not particularly interesting. Games wax and wane for years before becoming mainstream or dying off.