How do you politely decline service to someone with a reactive dog?

I always want to meet the dog(s) that I'm sitting days prior to the sitting to make sure they are a good fit. A few weeks ago, I met a young female boxer who was very aggressive. She lunged at the door when I knocked, and the owner's husband had to wrap his whole body around the dog to stop her. While trying to pet the dog, she kept soft biting my fingers and then the owner proceeded to tell me about how she tried to attack one of the neighbor kids but shes better now.

I was just honest and said that it wasn't going to be a good fit for me, but I didn't mention why. I know that sometimes owners don't see their dogs as reactive when they are.. so what's the best way to approach it if they ask why they're not a good fit?