Because they make tons of money from the in-app store. Valve has done pricing experiments on TF2 for years. Its retail price has varied from $20 to $1.99. They

Because they make tons of money from the in-app store. Valve has done pricing experiments on TF2 for years. Its retail price has varied from $20 to $1.99. They had already mostly saturated the market. Since they opened the in-app store a few months ago, they've made tons of money. TF2 now sells individual hats for up to $13, more than the retail price of TF2 during many periods. At this point, it becomes obvious: expand the market by making it free to play.