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.

NEW: Call of Duty: Modern Warfare 2024, Call of Duty: Black Ops Cold War, and Call of Duty: Vanguard are now available on Steam (PC).