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.

Home base. The PC version of Call of Duty returns to Steam five years after ditching the platform in favour of Blizzard's Battle.