Well now, that's a good question Ozark. And a logical one too.

I suspect it has to do with the thought processes of whoever developed the first muzzle loader sabot/bullet combinations. Considering the velocity limits of black powder and black powder substitutes, the rate of twist and velocities required to stabilize bullets of different weights and lengths, and the properties of the polymers used for sabots, some smart people probably concluded that short bullets in the 240 to 250 weight range would likely perform best. Those kinds of bullets in suitable calibers are typically pistol bullets. (Anyway, that's my guess.)

Here's a good question. Does anyone know what was and who produced the very first commercial bullet/sabot match for muzzle loaders?