I don't really find it surprising since it is like that with most crypto transactions. Bitcoin is still the king. Maybe the will make a darknetcoin ICO soon ^^
Most individuals still prefer using BTC for transactions, either for legitimate or illegitimate purposes because of the relative price security in terms of market cap. Users would not mind using an mixing/anonymization service instead of using privacy coins.
I read somewhere that companies like Ciphertrace that do blockchain forensics are able to track funds even if after they run through a mixing service. How legitimate do we think these mixers are for btc?
Chipmixer recently handled some stolen bitcoins and it seems untraceable so who knows.