why ransomwares are still problematic?
Public key cryptography lets the ransomware encrypt files so that only the malware author (who has the private key) can decrypt them or supply the needed per infection key.
Payment in Bitcoins is traceable, since all Bitcoin transactions are public. If the attacker transferred their Bitcoins to an exchange directly and converted them to a conventional currency, law enforcement could demand the records and find their identity. Mixing services allow the attacker to mix their coins together with lots of other legitimate coins, hiding where they end up. Mixing services exist and are only getting better.
The decryption for Bitcoins service can be hosted as a TOR hidden service. The attacker can set up a server that watches the bitcoin blockchain for bitcoin payments and provides one decryption key per payment. There is no link between the attacker and the servers except for the initial setup (which can be done through tor) and hosting fees (which can be paid in bitcoins).


All of these technologies are proven. It's just a matter of setting everything up and infecting victims. Until the general public learns to do better backups ransomware is here to stay.