“Hey, I will give you this trunk full of cocaine if you make sure to win one of my relatively unknown paintings at the next auction for a least $1m. My friend will bid it up that high for you to take the win. You probably can not resell it for that, but donate it to a museum and take a $1m tax write off for your trouble”

Money spent to buy the art is clean in one shot because art is worth whatever someone pays for it and it is hard for an investigator to prove you didn't simply -really- want that particular painting. Someone showing up to a bitcoin exchange with $1m in BTC trying to convert it to cash is going to invite law enforcement questions about the source of those funds really fast.

Art remains one of the most effective money laundering and tax evasion tools.

Cash is untraceable....? Although Bitcoin will reach that point as well