Haven't you heard about the surge on ransomware there are been since the Pandemic? Most of the times criminals managed to get computers with important information encrypted they always demanded their victims to pay Monero or Bitcoin for ransom (so they could get the encryption key and get their information back).
That is one of the most popular illegal uses of Bitcoin and alternative currencies nowadays, specially Monero.
I also think Monero must have an important presence within the world of trafficking of illegal substances but that is not something I have gotten myself out my way to find out, rather just common sense.
Also, since there are companies which are developing tools to track the origin of coins (specially within the Blockchain of Bitcoin), criminals feel less encouraged to use BTC to conduct their transferences of value and opt for more private Blockchains and tumblers/mixers.