By allowing everyone to add Steam keys for their products, free of charge.
This way, everyone jumped on the wagon, thinking to profit from the already famous and popular service... while in truth they just have unbelievably favoured Valve's monopoly and cut out almost every other choice from gaming on PC, shooting themselves in the foot. Now you either "get on Steam" or your game is worthless.
Just look at how many sites became just Steam key resellers, Humble above all, and tell me how many boxed games you can find without the Steam account requirement.

I don't get why you say that offering a promotional code with video cards and not allowing other resellers (and I underline the term "re-seller", not "seller", since almost everyone is selling TW3) is "rigging", while suffocating the market with that borderline criminal diffusion of Steam keys is not.

That makes no sense, that's all the publishers that did that not Steam.
They offered a service which appealed to the publishers, that's not the same.
And I think you need to check the definition of rigging, it's not an inherently negative word.

Maybe Im misreading this, GOG sold 40K and Steam sold 200K, yet you say GOG outsold Steam?

You are misreading it, TW2 sold way more copies on Steam.
That's why the OP is questioning the figures.
At least that's how I read it.

Ah, its a question more than an actual statement, OK, thanks for clearing that up :)

Of course. Valve has certainly handed over hundreds of thousands of free keys without thinking about their own best interests, not even for a moment. They are just generous like that!
Maybe you are forgetting that allowing this was a very precise and carefully studied business choice, one that largely repaid them in the long term; publishers decided to go this way, that much is sure, and in doing so they transformed Steam in what it is today... but claiming that Valve didn't expect precisely this outcome with their -brilliant- maneuver would be absurd, though. THIS would make no sense.
So no, Valve didn't achieve its position by chance, but consciously did that by exploiting the publishers' short sight, annihilating almost all competition in the process and tying them to their service. People now think that Steam is to PC what Microsoft, Sony and Nintendo are to their respective consoles. No wonders now EA does not sell games on Steam anymore, Ubisoft pushes Uplay and and Bethesda is trying to do something similar.

Edited to add a pair of sentences.