Because Donkey Kong gives more speed for exactly the same acceleration. I.e. you sacrifice nothing and gain something by switching away.
EDIT: If you mean that Rosalina looks slightly higher on the graph, I believe that's just a visual effect to allow multiple characters to occupy the same square - the stats are all integers.
> If you mean that Rosalina looks slightly higher on the graph, I believe that's just a visual effect to allow multiple characters to occupy the same square - the stats are all integers.