There is a chance that once the bitcoins have been converted into ‘real money’ or ‘real assets’ the ledger could leak information on the owners of those bit coins. But even then tracking and attribution can be very complex, but in answer to your question the reason in this case is probably that the attacker(s) haven’t ‘cashed’ them in yet. 

Depending on who carried out the attack they may never do anything with the bitcoin they have as they attack may not have been financially motivated.

There are ways to launder bitcoins using services such as Bitlaundry, Bitmix or Bitcoinlaundry.

These laundry services work as follows:  [(credit to the description below)][1]

1. Imagine that Alice wishes to send BitCoins to Bob.
2. Bob, sadly, is not well liked. Alice would rather not have anyone know that she sent Bob BitCoins.
3. So, Alice puts Bob's address in the form at BitLaundry.
4. Alice gets a one-time-use address from BitLaundry.
5. Alice sends the money to that address.
6. BitLaundry sends money out to recipients every 30 minutes.
7. (But, it doesn't send out Alice's money immediately, that might be suspicious..)
8. So, a random number of 30 minute segments later, BitLaundry sends the money out to Bob.
9. BitLaundry then deletes the database link between the one-time-use address and Bob.
10. Alice has sent money to BitLaundry, but people do this all the time. She's one of many.
11. BitLaundry has sent money to Bob, but BitLaundry has sent money out to a whole bunch of other people as well.
12. Alice and Bob are much less linked than they would have been otherwise.