According to Ciphertrace's report on Anti Money Laundering almost all drugs sold on darknet are bought with crypto. The University of Technology Sydney reports that 46% of illegal activity per year is associated with Bitcoin.
Not everything sold in dark markets (online markets on internet= darknet) is illegal because some drugs are legal in one country and illegal in others (cryptos make an easy payment.)
Illegal things also vary from tax evasion, money laundering, buying weapons to criminal offenses.
As for why BTC is common in dark markets? its value is increasing, not privacy.
One of the dark activities is hacking, many hackers use Monero currency.