There are two requirements to be considered a monopoly, or fall under antitrust laws:
Have a large market share
Be able to force competing products out of the market
Steam meets point 1, it doesn't meet point 2. On the other hand, things like the Apple App Store, don't meet point 1, but meet point 2, which makes them more likely to fall under antitrust. Windows meets both points, which is why the US sued Microsoft for not letting people choose their browser.