Because the grid needs a very large consumer to stop RIGHT NOW and be under contract to do so when told. To get this contract, the grid promises money. There is

Because the grid needs a very large consumer to stop RIGHT NOW and be under contract to do so when told. To get this contract, the grid promises money. There is no guarantee that adjusting the power cost for a customer will make them stop consuming power when it is needed. The contracts are in place to ensure the grid operator KNOWS that they can achieve stability when they need it.