It's the platform the majority of publishers chose to use.
Take Total War as an example. Shogun, Medieval and Rome were all released on disk. Medieval II had a release on disk and on Steam. And then with Empire, it was Steam only. You could still buy the disk. But it was basically just a Steam key and the install files.
I can't remember what the first big third party games were that moved exclusively to Steam, but it would have happened around 2007/2008. Within a few years, Steam was the primary platform for most games, with no alternative. At the time, the death of traditional disk installs was a big deal, only slightly sweetened by the fact that Steam's DRM was slightly less intrusive than some of the stand-alone DRM solutions of the time.
It's hardly the "people's platform" you're making it out to be. PC gamers had little choice but to adopt Steam.