Some people prefer bitcoin because of it's anonymity. They prefer not to be known who they are or what their details are. Some ask for it because of their own protection also but making it anonymous, makes it more dangerous. Better yet, ask for an escrow for transactions. Bad people prefer it because they can buy illegally and possibly get away with it, that's only my opinion.
But now exchanges need identity verification so now bad people cannot use it for illegal activities. They will know that their identity is already available with a specific exchange so with their address they can identify that what are they doing with their bitcoin. Just like coinbase can find that who is transacting to a gambling site.
That's what mixers and/or altcoins are for. No one with at least some sort of Bitcoin understanding will cash out coins from an exchange and directly use it to buy drugs or weapons in dark net markets. People firstly mix their coins through a mixer, and then convert their coins to an altcoin offering anon features. And then they either convert it back to Bitcoin, or they use that altcoin to buy whatever they want. It's not for nothing that Monero has been implemented in the dark net market AlphaBay.