When Valve launched Steam it offered smaller developers a place to showcase their titles to a larger audience while simultaneously providing anti-piracy DRM that smaller developers with few coding resources could not afford to develop themselves. There were many competitors at the time (2003) and several were quite popular. I believe the reason Steam won out, and this is entirely my opinion, was that their DRM was invisible. I was an early Steam adopter (I also used many of the popular competitors) and had no idea Steam had DRM. Almost every program with DRM I have interfaced with has had DRM that is in your face in some way. A lot of DRM systems must be connected to the mothership to work. If you have flaky internet or the mothership is unavailable...too bad!
EA got cocky and launched Origin and pulled most of their content from Steam. They had no idea how hard maintaining that system would be and just how much people actually like using Steam. Long story short they eventually modified Origin to link to Steam so they could list their titles on Steam again. The "If it aint broke, don't fix it" adage kinda fits here.