I feel like people have often downplayed the Banjo franchise. Yeah, it's collecting dust nowadays, but during the N64 era it was an iconic, prestigious part of the Nintendo/Rare partnership. The original Banjo-Kazooie was a highly anticipated game within the industry, getting tons of hype and coverage before it even came out. Despite the status of it nowadays, after K. Rool and Ridley got in, if Nintendo and Microsoft were both made well aware of the fan demand, there would be little conflict bringing them to Smash.
It's actually pretty similar to how Mega Man was added to Smash 4 despite the franchise being put in cold storage at the time.
And I agree with Ryce that this is a special case for western characters. I fully expect the last two to be Japanese.