http://www.washingtonpost.com/blogs/the-switch/wp/2018/06/20/why-bitcoin-exchanges-keep-getting-hacked-and-how-to-protect-yourself/

The price of bitcoin took a tumble early Wednesday after a major South Korea-based cryptocurrency exchange, Bithumb, admitted hackers made off with more than $31 million worth of virtual currency. The incident is the latest in a long string of thefts at the online portals where investors trade cash for digital coins such as bitcoin and ether. Bithumb has not said how the attack occurred.

What makes exchanges vulnerable to these sorts of attacks in the first place?