Why do you think a lot of people who play one of these games play the other? Is it mainly just the trade?
Spiral Knights and Team Fortress 2 Connections?
For me I saw a few free games and downloaded them. After awhile I realized that keys could be very efficiently traded for E.
I came here from TF2 as well, looking for the hat. I actually played a few years back for the Steam Winter Sale achievement hunt, but it was only in 2014 that I started actually playing
Didn't really play TF2 until 2013 or so, started playing competitively last year. There's never really been a connection between the two for me, anyhow.
As mentioned, a lot of TF2 players started playing SK after getting the hat from the promo. I happened to start playing SK the very same day SK was released on Steam and you could get the hat, so I stumbled upon a completely filled rescue camp with loads of people going for the hat; I was clueless.
Then there's trade with keys as well, but you can trade for Dota and CS:GO items too.
I dont remember if i played SK before TF2 but like the only hat i wear in TF2 on like 60% of my classes is the knight helm.
I started playing the game to get the TF2 hat. As of now, I've clocked in over 5x as much time in SK than I have in TF2.