As Bitcoin saw a huge leap in price last year, many of its expected (criminal) users are now using the cryptocurrency a lot less due to law enforcement being able to uncover transaction patterns.  These users are instead choosing other Bitcoin alternatives, such as Monero and Zcash, which have no tracking for those preferring anonymity.

Bitcoin is not as anonymous and encrypted in its blockchain data, specifically showing time and amount - - the inflow and outflow can then be monitored to track down criminal activity.  Bitcoin addresses and wallets are under close surveillance, becoming high-risk to deal with.

Monero, which grew over 3x at the end of 2017, has been able to offer privacy making it extremely high-risk and the cryptocurrency of choice when it comes to ransomware attacks.  

Zcash offers even more privacy by encryption of address, however, tools have been created that help monitor transactions of this type.