Unlike the Charger, the Challenger will require significant changes to the platform which costs more money. Even if the upfront design and infrastructure changes cost only as much as those for the Charger, they would also have to sell as many in order to get the same benefit from their investment. However, if it really is the first of a new, more flexible "LY" platform, then the extra cost won't matter much in the long run, since the other cars will eventually benefit when they are redesigned.