One probability of the drop is because they are using other cryptocurrencies based on anonymity like Monero, Zcash, Ripple as tracking the transactions with this altcoins is much harder compared to what we see in the blockchain of BTC. The shift in cryptocurrecy used in crimes is pretty prevalent as already the exchanges in South Korea have delisted the aforementioned cryptocurrencies as per order of their government.
Privacy coins are taking the attention of the criminals but then when the media cover a story, the only cryptocurrency they would mention most of the time is bitcoin. If it is about an investment scheme then they would immediately say "bitcoin scam" that's why people always assume that bitcoin is
the scam itself.
The South Korean government has mentioned that millions has already been laundered with these coins and they hope that the delisting will help lessen the attempts made through these coins, however they still are figuring a way on how to stop it completely.
I don't think that they can completely prevent it though because people will always have a way to achieve what they wanted. You're right that the criminals shifted their attention more to privacy coins as bitcoin is not that anonymous compared to the coins you mentioned that's why it is not the choice of most criminals anymore. But then every time there is a news, bitcoin is still the one being pointed out as used by criminals because many people only knew bitcoin and thought that it's the only cryptocurrency existing.