Rare had to bundle to the N64 Expansion Pak to get the game to work.
Gamers who chose the Nintendo 64 over the PlayStation, back in their day, might recall upgrading the N64 Expansion Pak. Released in 1998, it was essentially a memory upgrade that doubled the N64's main memory from four to eight megabytes. Some developers would use the extra oomph to do things like improve a game's visuals. Others classic titles like The Legend of Zelda: Majora's Mask actually required the expansion pack to run.
The first game to rely on the presence of an Expansion Pak was Donkey Kong 64, released in 1999. The game came bundled with the upgrade, and while advertising would insist it was simply too big (and presumably awesome) to operate without the upgraded memory it has now been revealed that wasn't quite the case. "There was a bug that caused the game to randomly crash, and only occurred in the 4MB only version," said Chris Marlowe, a programmer for Donkey Kong 64, speaking on a Conker's Bad Fur Day commentary. "They couldn't find out what it was so they had to ship with the [Expansion Pak] in it."
The inclusion of the Expansion Pak had less to do with the game's size or complexity, and was rather the easiest fix its developers at Rare could devise to stop the game-crashing bug. Easy fix or not, the inclusion of the Expansion Pak, according to Marlow, would "cost [Rare] a fortune." That being the case, it might have been a bit of serendipity for Rare who would go on to release Perfect Dark, another N64 classic that required the Expansion Pak, the next year. Because of the Donkey Kong 64 bundle, there was a decent consumer base already possessing the handy little upgrade.