According to the analytics firm, one reason for the large decrease is that total cryptocurrency-based economic activity grew three-fold from 2019 to 2020.
Another factor is that Chainalysis underestimated the number of active addresses linked to criminal conduct in 2019. That's because so much of the criminal activity that occurred in 2019 wasn't identified till later, including activities connected to the China-based PlusToken Ponzi scheme.
In July of 2020, China's government said that it had finally arrested all 27 major suspects alleged to be behind the multi-billion dollar scam.
Chainalysis notes that 2020’s current crypto crime statistic of 0.34% could increase over time for the same reason. Still, the firm emphasizes that crypto crime is decreasing as a portion of the total crypto economy.