The Great Video Game Crash of 1983/WMG

It was caused by a massive explosion of horrible.
In 1983, Atari had just put the finishing touches on a brand new, revolutionary game that would make them king of the market — Atari's Ultima, which despite being for the 2600 had amazing graphics, sounds, gameplay, etc. Boxes and booklets were printed, cartridges manufactured, and print advertisements drawn up. Atari had a large booth reserved just for Atari's Ultima at the Winter CES, planning to both show the new standard in gaming and make Activision look like a bunch of poor-quality hacks by comparison.

But, as things often do, the combination of the terrible ShaqFu and atrocious Kazaam caused a massive explosion in space-time, cracking the universe and erasing thousands of good things — Nikolai Tesla's inventions, Love's Labour's Won, Hemmingway's suitcase, etc. But it also time-nommed Atari's Ultima, thus causing the massive video game crash of '83.

To prevent this happening, Atari invented a time machine and travelled to 2001. They sealed Shaquille O'Neal in a giant metal prison, doomed to be trapped forever, then created a plastic replica of Shaq to stand in. Despite this, Atari's time machine broke down and they were trapped in 2001...and the explosion still happened. Thus. Atari was doomed.

It's Custer's real revenge.
Yes, had to say it. When the games raped our minds that much, it's got to be that.

If the crash didn't happen, Nintendo wouldn't be a major gaming company in America.
Nintendo was such a success because at the time there was nobody to steal sales and so they were unopposed. If the crash never happened and Nintendo still tried in America, the latter would be up against the powerhouse Atari (with its partner-in-awesome Activision). All contenders would continue improving on the overall quality, and going up against an established brand would cause Nintendo's sales to be mediocre in America...relegating them to being an also-ran.
 * Unlikely. Look at the other side of the Pacific -- Japanese marked never had anything even distantly resembling the Crash, and the Big N still reigned supreme there, despite the fierce competition from the vastly technically superior platforms like MSX (which was a Japanese answer to Commodore 64). It was their quality games that won the war for them, not a market situation or superior tech.

If the crash didn't happen, Nintendo would still be a major gaming company in America.
On the other hand, we need but look at the battle between the Atari 5200 and the Colecovision. The former showed off Super Breakout, while the latter showed off Donkey Kong. Yes, that Donkey Kong. Nintendo was already planning to release the Famicom in North America, and they still had exclusive rights to the sequel of Mario Bros. — Return Of Mario Bros.-er, Super Mario Bros.. Essentially, the same thing that happened in Japan to the MSX would happen here. Nintendo's console would have won out against the 7800 because it had the ability to scroll the screen smoothly, an essential element in almost any game made since.