What's so funny? GW1 is there. :S
Valve has updated its policy on DLC sales after GW1 release. Any DLC that were sold outside of steam were 100% profit for the publisher. Nowadays, steam games must handle dlc through steam, and Valve takes a cut.
And since ANets business model relies 100% on cosmetic dlc once you've bought the game, I'd say the chances of them giving Valve a cut on every sale for the rest of the games lifespan, just so we can download it through steam, is rather slim.