In the
recent dev interview for Bananza, Kenta Motokura (producer) states:
This is was a funny observation on their part, and it's funny because it's often true. For instance, I never found it appealing to play as DK in Mario Kart because not a whole lot of what makes him recognizable is reflected when you control him.
With other characters, you still see core parts of their identity when you view them from behind. The bros. have their caps, the princesses their crowns, Bowser has his spiked shell and red mane, heck, other Kongs have striking pieces of attire. With DK, his main draw is arguably the tie, which is mostly hidden away from the player. What remains is a "mass of brown fur" that is pretty boring to play as in a 3D space.
DK also has a remarkable primitive side to him that is lost in Mario Kart. He is kind of designed to be more animal-like than other characters, even other Kongs, and move around by dragging his knuckles. Bananza even retains that side of him, which I think counterbalances the lack of other immediately identifying features when viewed from behind. Putting him behind a wheel inherently strips him of this trait and just kinda renders him as a generic heavyset, hunched humanoid.
This is why I've always preferred playing as other Kongs in Mario Kart despite being a fan of DK.