We have been seeing this kind of accidents from the very beginning that a exchange gets hacked when it reaches a specific amount of money.
Most of it happens when people started to trust and then boom it get hacked.
I do not know why people still trust some exchanges with that much money.
It is the biggest truth of technology world that no system can be hundred percent secure and those that hold high values are the major targets of criminal hackers. Moreover, it is not like the hacker woke up one day, made few clicks and goes down the website or exchange. A lot of days, even months might get spend to make one hack reality. The best is to choose exchanges that take responsibility of such compromises.
Obviously hackers wait and analyses the websites very closely and waits for only one mistake to happen and then they do their attack.
What I was saying earlier is that you just keep your coins secure on some cold wallet or at least in your own wallet and never keep coins on exchanges, Who knows when they can get hacked?
Regards
TEX-LXRY