Any currency can be used for illegal activities. Why the dollar is the most popular currency for these purposes?
because dollar is the most common currency in the world.
In general, bitcoin has more prospects for use in this area, because of anonymity
Well, this is true, i agre with you.
Bitcoin sometimes perceived as the (wild west of finance), the cryptocurrency space has been connected to various scams, hacks, terrorist and other criminal activities which have taken advantage of this anonymity.
In the fact, almost the transactions of darkweb stopped or reduce in Bitcoin.
I heard hackers and people of underground world are using other cryptocurrencies for their transaction instead of Bitcoin.
Monero, Zcash are the best choice for them, you can see the price of Monero increased a lot of values recently, that is the reason.