Seems very unlikely. Are their fees bullshit? Yes. Is Steam a monopoly? Sure. Have they achieved this through anti-competitive measures? Absolutely not that I'm aware of.
It's largely that they were just the first game store but it's also that they are continuously the best game store, on all fronts. While other game stores are tripping over themselves to make the experience shittier, unable to complete the simplest of tasks like logging in or launching a game.