First of all, everything involve criminal activity, from the plumbers to the governement itself, to the currency or crypto currencies.
Now, Banks do not like crypto-currencies "high risk of money laundering", crypto is a good way to move funds easily and fast from two points, but remain the same issue: In/Out for cash out let traces is where the Banks will obviously catch them.
They create fake problems, and give a big regulation. Classical.
Why bitcoin instead of monero & zcash ?
A lot of darknet market use/accept monero and even ether!
I guess is prob a Liquidity issue, and that also bitcoin is the most well know crypto