As per my knowledge of scooby doo episode's their are few episode where Scooby and Shaggy said they don't want to lead the monster to the trap. So they have to choice someone else but no other member got agree and they again used scooby snacks to bribe Scooby and Shaggy.
So to me the reasons are-
- Nobody else from the team want to do this
- Scooby and Shaggy can be easily agree with the bribe
- They have more expertise in leading monster to trap
If you analyze then you will see their is a proper task distribution among scooby gang as per their skills. Daphne Blake can never be a choice because her focus is on makeup and she will never volunteer for it, similarly other characters are also have expertise in their work.
I think their are some exceptional episode but don't have link to them.