Also, not claiming this is definitely the case here, but some people might rather pay a higher fee and have a higher effective mixing then pay virtually nothing and be easily found on-chain by anyone. After all, this is the entire point of using a mixer: privacy.
Well, yes. I agree with that. If I really want to hide my money from the people, I would pay more fees to have better anonymity. If you care about your privacy and your money, you may spend a hundred bucks to make it as you like. But, there are people like me who are neither a newbie nor an advanced level user, we usually prefer lower fee mixing that provides some privacy.
However, advanced users who have hefty amounts and want better privacy may try the service with higher fees because they want better privacy! To be honest, I don't fear anyone finding me on the internet by tracking transactions. But, I don't like to be tracked too. In this case, I may choose lower fees mixing.