You know what else is popular with criminals?

Walls.

Mailing using envelopes instead of postcards, especially those envelopes with the ink patterns that make it harder to see through them.

Not yelling things into a cell phone at top volume when you're out in the street.

The First, Fourth, and Fifth Amendments to the American Constitution, and equivalents elsewhere in the world.

Cash! Dollar bills are like anonymous physical bitcoins!

Bitcoin but without the audit trail! Now who could possibly want such a thing besides criminals...

Bitcoins are about as anonymous as a numbered bank account are, if that.

Right. That's why cash is like anonymous bitcoins.

"Mailing using envelopes instead of postcards"

Which is why there is a program to track the envelopes of all postal mail.

I can't think of a measure for "popular with criminals" that wouldn't also include the FBI's purported tool of choice.