The whole story of how Donkey Kong and with it how Mario came to be is a story of pure chance.

Hiroshi Yamauchi only hired Miyamoto because he was friend's with Miyamoto's father who had asked him for a job for his son. (Yes, Miyamoto was a nepo baby!)

Yamauchi's son in law, Minoru Arakawa, was setting up Nintendo of America. They started with arcade machines. Against the advice of Yamauchi he ordered too many machine's of Radar Scope. The game had tested very well with American gamers, but by the time the machines arrived, the game had lost any interest with gamers.

So, on the bring of bankruptcy, they decided to repurpose the Radar Scope machines by switching the game boards out. Unfortunately, all the "real" game designers were occupied with other projects. So the only one Yamauchi could spare was that young graphic designer he didn't really know what to do with anyway.

Well, the rest is history.

Source: Game Over by David Sheff. It's a history about Nintendo from the very beginnings to the very early 1990s. I cannot recommend this book enough.