Because Mario was bundled with the NES.
That resulted in massive exposure. Back then, for most kids, quality games were few and far between. Yeah, some rich kids always had a ton of games, but most people only owned a handful. Mario became the default game that EVERYONE played. That didn't just mean exposure, either, it meant that Mario came to define the taste in gaming for many, many people. It was the first game for many, and the most played game for many. The result was kind of predictable.
It's not even like the first Mario games were even that great, in my opinion. I barely played my copy, mostly because of a local rental store and because I had a friend who had a bunch of games. I found tons of stuff I liked better, from Ghosts n' Goblins to Nobunga's Ambition, so Mario never settled in with me as a main component of gaming.
I've been saying for a long time that if any of the Big Three wanted to grow a new IP to massive proportions, they should bundle it with EVERY console, standard. It might mean less short-term profits but it would pay off long-term. Of course, it wouldn't ever reach the same magnitude because of all the options in gaming now, unlike when the NES was the only game in town, but it would help.
I will always believe that Mario would never have come close to its current popularity had it not been a pack-in. It's kind of a shame, as I would much, much rather that Kirby had been given the role of Nintendo mascot.
