Most of the playable racers are classic Kirby enemies, not his allies or new characters. Sakurai reported that it is easier to start designing a rider from an enemy than from scratch.
The reason is simple: in the world of Kirby, the ability copying mechanic gives each enemy a unique ability – someone shoots lasers, someone flies, someone weighs a ton. These characteristics directly affect the riding style and are easily read even during a quick race.
At the same time, friendly characters, like Waddle Dee, are devoid of special abilities, which preserves their charm. However, important main characters for fans have been added to the roster.
Also, using ready-made enemy models saved time and expanded the list of racers without postponing the release.