IMO, it was down to the death of PC games in retail stores. For years PC game shel space was getting smaller and smaller until in the mid 2000s, it was practically non existant aside from The Sims and other shit. Suddenly, with steam, PC gamers didnt need retail anymore, and here was all the games you could ever want, available to download, at prices cheaper than retail. All patches, updates, etc were delivered automatically, all your games were neatly organized under one account, on one program. Im of the firm belief that if steam hadnt come about when it did, PC gaming wouldve all but died out. Now its in its renaissance and its fucking glorious