Consider the value of bitcoin,it has made exchanges prime targets for hacking, and thus has made hacks there quite lucrative..
I mean the anonymity and irrevocability associated with bitcoin transactions means that the money lost is also irrecoverable, CMIIW..
Well bitcoin is unhackable atleast till today's date but who news in future with quantum computers coming it might not be unhackable as its is now.
The other thing is users/exchanges gets attacked and hacked due to their loopholes in the security. And bitcoin being anonymous you can't track the user that is right.