Oh, I see. This should explain it (from the
trading cards community FAQ):
What about free to play games like Team Fortress 2 or Dota 2?
Free to play games drop game cards based on your in-game purchasing.
For every $9 USD spent (approximate) since the start of the Steam Trading Card Beta, you will earn one card drop. This card will drop at some point as you play. If you have made in-game purchases in Dota 2 or Team Fortress 2 prior to the Steam Trading Card Beta, or if you owned Team Fortress 2 prior to the Free to Play update, you will have 4 or 5 additional card drops available.