It's because SRB2Kart Character mods are leagues easier to make than a standard SRB2 character mod, especially when it's done from scratch. There are 87 sprites used for Sonic in SRB2Kart, and a good amount of them can be substituted or modified by... For example using a distortion effect or slightly tweaking preexisting sprites. Sonic in standard SRB2 takes 212 sprites, plus another 134 for a super form. That's almost 4 times the amount needed for SRB2Kart, and since many of these sprites are dynamic and are supposed to characterize numerous actions that don't revolve around a mostly static position such as driving a kart, it can't really work with the same basic shapes for their composition in that regard. So with that being said, it isn't really much of a surprise that SRB2Kart characters would be more of a popular choice to make.
