Most likely the people running sex-trafficking rings are not stupid enough to use something like Bitcoin as you don't want to have a public ledger with all your transactions fully visible on the internet if you're doing stuff like that.

Probably Zcash is a more likely contender for that, as it is the only cryptocurrency offering fully anonymous transfers together with normal, public transfers in the same wallet.

In what way would having all your transactions publicly accessible to everyone on the planet be the best way to run an illegal business?