Well, back in the day, Jim wanted to keep certain performers exclusive to certain ends of the Muppet spectrum, allowing himself, Frank, Jerry, and Richard to be free to perform in whatever came about - Steve and Dave were among those Jim wanted to keep exclusive to the Classic Muppets, which is why they rarely performed on SST to begin with (I think the only times Dave performed on SST are Elephant Elevator Operator and Tiny the Giant Chicken). Really, the main reason Steve got to work on SST was because he took over Kermit, and by proxy, Ernie, since Jim's passing.