The answer is pretty simple, because bitcoin anonymous, you don't need any id or any type of identification or personal information to get your hands on bitcoin, so it makes it easier for anyone trying to hide their tracks and escape from paying taxes to the government, or launder illegal money, that is why bitcoin is being used by all the hackers and drug dealer on the black market, and that is what government have against accepting.
Bitcoin isn't anonymous. It's important to understand that once your transaction is confirmed, it's stored in the blockchain for ever. Initially you might consider yourself safe, but at some point the link to you CAN be made.
Fiat as in conducting cash transactions in that regard is a far better way of remaining your privacy, and thus a better option for criminals and whatnot. No logs or transaction history, nothing will ever point to you years later.
It's a frequently debunked subject, but it keeps being used as an excuse to put blame and shame on Bitcoin, which should stop for once. Cash is by far the main root of evil when it comes to all things Bitcoin is being blamed for.