[jhanschoo]

This is all from memory, so I may misidentify some factors as important and fail to identify some factors, and my chronology may be wrong.

Steam won because it had first mover advantage. Valve was historically a developer, and it launched Steam with a highly-anticipated game, so I suppose it didn't have pre-existing relationships with its retailers to negotiate. Other traditionally to-retail large publishers didn't get into the online distribution and online DRM game till quite a bit later. Meanwhile, the indie gaming scene exploded, and many of them chose to distribute via Steam.

As for why it continues to be the most popular platform, I presume that they are mostly due to it's reach as a channel as well as its gamification of its platform to encourage loyalty and spending. I think I should let someone who's seen things from the developer side answer this though.