In any case, with regards to darknet safety, the darknet users should use Monero to secure their privacy. They do not want their transactions made on a blockchain that everyone can follow and analyze. It might put themselves in trouble later.
I haven't ever used a dark net market, but couldn't it be that vendors have the freedom to choose whatever currency they accept as payment option? If that's the case, then it might be that they prefer Bitcoin over XMR.
If the vendors you buy from prefer Bitcoin as payment, then people don't have much of a choice but to use Bitcoin. If it was something the market itself decided, then they are way better off using solely XMR or other privacy coins.
At the end of the day, Bitcoin has the biggest network effect, brand recognition, acceptance, and so forth. That apparently is reason enough for people to keep using Bitcoin and sellers to keep accepting it.