Why are dentists so reluctant to put clear braces on the bottom arch?

I see people with clear braces but they have metal on the bottom. I heard that that is because the brackets on the bottom might wear away the top teeth. But doesn't that depend upon your bite? But, I have noticed that people who have Six Month Smiles braces have clear on the top and the bottom. Does Six Month Smiles have some sort of mechanism to prevent the upper teeth from being eroded by the lower? Thank you for your time.