Instead of being a true rival to FIAT, it became a thing with a toxic-as-hell commumity, fraud, and basically its nothing more than a high risk stock. The risk is NOT only "will this go up or down" but you have a high risk of being robbed, as have happened to millions of people.
Maybe there will be a better alternative in the future, but right now bitcoin is not it.
Ransomware would still work just fine using regular bank transfers. Especially given that the payor has no incentive to stop that money from arriving at it's destination.
But sure, using crypto the criminals get to keep the 20-30% they'd pay for payment processing otherwise. I'm not sure that really makes a difference though.
Banking trojans were doing just that, and receiving that money is obviously far more challenging than ransom payments because the owner will tend to notice pretty quickly and want it back. Ransomware doesn't have this problem, they can just deliver the keys only after they actually have the money in their control.
BEC groups are stealing billions every year via bank transfers right now.
.ru crime forums are absolutely full of people who will handle the logistics for you. They'll provide you bank accounts to send the money to, and deliver you whatever is left after their cut. A regular customer with high volumes will get excellent rates. The logistics aren't a challenge because there is absolutely massive pre-existing infrastructure already available.
Ransomware isn't big because of crypto(currency), Winlockers were popularized well before crypto payments became common. Ransomware just happened to develop at the same time as crypto, not thanks to it.