From a content standpoint, the original PlayStation brought a wider variety of titles and genres to a wider audience. If there were one "untapped" major territory left to exploit, it was North America. Yes, Americans have been gaming since PONG. But it was the PlayStation that presented console gaming to an older demographic in contract to the younger audience of the previous console generation.
Literally the entire world except the US and Japan were untapped. I'm sure there's at least one major market in there somewhere.
Besides, the charts posted earlier show that console sales in the US never even increased that much with PS. Looks to be about a 22% increase over the 4th gen, which is actually the smallest % increase of any gen on that chart. https://gamrconnect.vgchartz.com/post.php?id=9178405
Plus, couldn't you argue that people from the previous generation getting older also meant there was now a bigger, older demographic?