Yes, Bitcoin seems to be popular in the trade of illegal goods and for paying ransomware. Doesn't exactly explain why most other people invest so much into Bitcoin. And the security is very dubious, considering the track record of Bitcoin exchanges.
People have wanted an online cash equivalent for decades. It's a privacy thing. "Illegal" means whatever authoritarian jerks want it to, so we can ignore that.
Ransomware obviously uses Bitcoin because it's reliable, and can be anonymized. And people use Tor for child porn and "illegal" drugs because it's secure and ~anonymous. They're popular because they work. And in a way, they're canaries for decent privacy lovers.
Sorry, thats just a strawman. Bitcoin enables sending money over the internet off official channels. You are not going to pay for ransomware with your credit card.
How do you pay for ransomware with cash? Put it into an envelope and send it off to another continent to the address of the criminals? I explicitly talked about sending money over the internet.
you pay ransom with cash. because cash has certain properties. which bitcoin also has. which is why bitcoin is being used as digital cash. which in itself isn't a problem and if it is - you need to fight against cash, there's orders of magnitude more illegal stuff being paid for using cash than using bitcoin.