First off, developing new cores is kinda pricey. Good idea to mess with that as little as possible from a money standpoint.
But beyond that, it's all about matching up, right? Some bowlers may want the same core with a pearl cover, other bowlers may want the same core with a solid cover. What if you like the type of motion you get from a core, but want to get different reads on the lane for different conditions? The same core with a solid/pearl 1-2 punch. Why do you think so many people are excited about having both a solid AND a pearl version of the Cell?