Why can’t the bank account of the extortionist be traced?
There is no bank account for bitcoin
Bitcoin exists in the form of a shared ledger in which all transactions are recorded and freely viewable by anyone. (Just check out blockchain.info for example)
The catch is that nothing inherently links a given public key (used to send bitcoins to a given account) to a real-world individual. With a lot of detective work I expect most criminals trying to use bitcoin will eventually be caught, but it will take time. Criminals are having a hard time over the long run keeping their bitcoin activities secret, since even one slipup that connects a bitcoin transaction to an identifiable source gives law enforcement a solid lead to follow.
Current price of a bitcoin is ~$330 at the moment, BTW. That’s in the same ballpark as typical ransom demands. I really hope some solid countermeasures can be taken against ransomware as it makes people hate the method of payment (bitcoin), prejudicing them against a revolutionary advance in finance.