As of late, crypto has become extremely popular due to the negative effects of COVID-19 in the mainstream economy. People are starting to consider crypto to be a safe-haven against the traditional monetary system empowered by banks worldwide. Yet, the vast majority still relies on Fiat for day-to-day transactions because of its price stability and widespread acceptance in the mainstream world.
With the advent of privacy coins, people can enjoy true financial freedom like never before. Privacy coins can become attractive tools for illicit activities because of their untraceable blockchain ledger. Despite all of the benefits privacy coins provide, criminals still rely on Fiat for all of their nefarious actions. I'm starting to wonder why, since Fiat (physical cash) is not as anonymous as one thought it would be. Privacy coins are truly anonymous (especially those that are private by default like Monero and Grin), as there is no central authority behind them. Maybe criminals prefer Fiat because of its high level of adoption in the mainstream world? Or maybe because Fiat has an easier learning curve than crypto?
If by any means criminals start adopting crypto in mass (especially privacy coins), we could expect a full government crackdown on the entire industry. Which is why I think it's best for criminals to continue using Fiat for the sake of the crypto/Blockchain industry's growth worldwide. But we'll never know what will happen in the future. Thoughts?

We're talking about different classes of criminals when discussing fiat and cryptocurrency transactions. The local drug dealer isn't inclined to accept cryptocurrency because they are running a cash business - they buy in cash, they receive cash and they restock in cash. Besides that, their customer base is unlikely to be super sophisticated to use Bitcoin, or at least enough customers don't do it to make it worth additional hassle. The type of criminals you see talked about in the news and referring to crypto are large scale - randomware attackers, money launderers and criminal syndicates that generally just need to move money across borders with ease.