Because Modern Warfare is published by Activision, which is a part of the bigger Activision/Blizzard company. So to save money, they just put it out on the Blizzard launcher (Battle . net) instead of having to pay Steam to sell it here and they keep a part of the profits.
Expansive Customization. Fine-tune over 500 individual settings on the PC version of Modern Warfare II, from keybindings and the ability to play on a controller, to graphics, audio, and interface sliders and toggles.