I've never checked out dark web, but I'll use some common sense / logic. There are more directions here:

1. I expect that some of those "deals" are fake, like everywhere in the world. So obviously a mean of payment that's not reversible is preferred by the sellers.

2. Since the things sold/bought there are usually (?) illegal, linking them to somebody's name, address, face.. are clearly avoided.
And then the next question arise: how comes they don't use anonymous coins and risk (just a little, I know) to use Bitcoin?