How do I get a wedding deposit back when the wedding is being called off

Depends on the terms of the contract. In some cases the deposit is non-refundable. In other cases, a portion can be refundable depending on how much time is involved prior to the scheduled date. In other cases, it may only be refundable if the place can be re-booked.

So you need to read your contract.

Scott is right - many of those contracts are non refundable and are written because of people who have called off a wedding. The person (company) you booked a service from has already made plans, possibly invested some materials, perhaps turned down any other business for that day, and that person or company would want something for their efforts. Just business.