Also, he's on a quest to find DK... Diddy is literally on a Kong Quest.
In many Donkey Kong Country games Cranky Kong tells you that old games were much more challenging than they are nowadays. First I thought it was quite weird, since he was never playable. Then I realised that nobody needs to be the main character to PLAY THE GAME HIMSELF.
Actually, Cranky Kong is canonically the original Donkey Kong from the arcade game. The Donkey Kong in the DKC series is the original Donkey Kong, Jr. When Cranky is talking about himself and referring to older games, he's talking about his appearances in the arcade cabinets (and the original Game Boy remake).
In Donkey Kong 64, you play the original Donkey Kong arcade game for one of the Golden Bananas and the Nintendo Coin. Now, Jumpman, the character you play as in Donkey Kong, moves slow and can't jump high... sort of like the Kong you rescue in the same level.
Imagine if the Popeye game went through ... the whole Mario series would have ceased to exist.
We all know that this Donkey Kong later became Cranky, and was beaten fairly easily by Mario. You probably also know that he went berserk in the first place because he was a pet that Mario abused. Flash forward to Donkey Kong 64, where this Donkey has been Cranky for a long while and has lost his physical strength. In exchange, however, he's gotten smart. He's a master Mad Scientist who knows how to manipulate and enhance the body. Mario is used to dealing with strong, hulking opponents such as Donkey Kong and Bowser - he likely doesn't know how to deal with someone in a completely different area of expertise, let alone if that someone would be rightfully pissed off if he ever saw him again. At best, an encounter between them would be a Humiliation Conga. At worst, a Curb-Stomp Battle.