Because tourists are easy victims. They are not used to how things are done in this place, they are lost and not knowing where anything is, and they might not even understand the language that is used there. Parasites and predators go where the easier victims are. Even things as simple as the tourists not knowing what the usual price is for things, they can sell the same thing to a tourist for a much higher price.
I saw a picture from somewhere in South America where tourists sometimes go. The guy had a sign that said "Orange juice $10" and another sign that said "jugo de naranja $1" These are the same thing.
