"Why has nobody mentioned that cryptocurrencies are a blessing for organized crime."
Because you overestimate how useful it really is to them and also underestimate how useful its alternatives are. Let's take each adjective in turn:
"Untraceable,": Nope. The most popular ones, E.G. Bitcoin are very traceable and used so often that there are already automated programs specifically for tracing transactions. And it's not just law enforcement which has that. You can go trace it for fun if you like. Some other ones are much less traceable, but they're harder to exchange and spend. Now, compared to real currencies, a lot of crypto is more traceable. If I transfer cash, you probably won't find it again. Unless I stole an easily-numbered bunch, the chances of you knowing which notes are good and which are bad is low. Even if I did, I likely get to spend it before someone scans the numbers.
"easily transportable.": Crypto transaction fees make transport more expensive. Meanwhile, for most organized crime, the organization has already figured out the cheap way of moving money. It's core to their business. Given the large number of fraudulent bank transfers, banks willing to launder money for criminals, and similar structures, moving other currencies doesn't appear to be very hard.
"Can be used to launder money,": Not really. Crypto is like money in most respects, meaning if you suddenly have a bunch of it and make that known, people are going to ask how you got it. To launder money, you need a good excuse for the legal method you used, which is why a cash-based business works so well. "I got a hundred grand worth of Bitcoin yesterday" is not such a good one.
"transfer money cross borders,": Well yes, it does that, but you can already do that. See also easily transportable above.
"pay for contract killings, [list of crimes]": Right. Like everything else. It does not help your argument to list a bunch of criminal activity that you could pay for. If I said that the U.S. dollar was used to pay for all those things, plus government corruption, environmental destruction, [editor's note: this sentence would take about ten minutes to read. This list has been cut here], it still wouldn't be a reason to prohibit use of U.S. dollars unless your alternative was better.