Why Wallet transactions cannot be tracked?

I recently heard that virus program distributed among the android users and they encrypt their data and ask money(Through a wallet address) for recover them,, the thing is transaction cannot be tracked? why?

Is there any court cases against bitcoin? why do Europian Bank say bitcoin does not involve traditional financial actors? 

I've heard many of my companions say the bitcoin is the greatest invention in the decade.. But I wonder why every country don't accept this as a legal currency in their general transactions?