Why can't you put dish washing soap in the dishwasher?

Dish liquid is designed to generate a lot of suds. A dishwasher cycle will stir up excessive suds that the machine won't be able to rinse away. Some dishwashers will stop or break down if this happens. If you're out of the regular dish washing detergent, rinse the dishes well before you put them in and then run the dishwasher without detergent. The hot water and dry cycle will sanitize dishes sufficiently. Or, put the dishes in, close the door and leave it until you can get some detergent.