Because other calipers are designed completely different. They have the seal in the bore (a square o ring) and a smooth polished piston (so much easier to make), it's all about that V lip seal. After the corvette calipers GM wised up and went the conventional route. The corvette was just early when it came to disc brakes and after the C2 GM just carried it through. When for instance you bought a C3 in 82, did the brochures scream handing and braking or did they say brake system designed in late 1950s with flawed lip seal issue whenever you have a rotor runout problem (and still today, most shops don't check for runout on cars) or how about a not state of the art 1950s flared toe oversteering trailing arm suspension? The C4 was a BIG leap forwards but still, lots of bean counter influences there made a good system a little less good. The c5, that's when they really wised up although the transverse leaf still is viewed upon with scrutiny (but GM uses it for good reasons, like anti roll properties in the dual <- keyword!!! mounted cambered leaf springs) and the space benefit
