that is a very strange move by the hacker! there are already many ways to exchange coins with each other and there is also mixing techniques that could be used to make it harder and even impossible to trace the coins!
I read that most of the assets were ERC-20 tokens -- probably very hard to mix due to lack of market liquidity. I imagine the hacker was hoping to quickly wash the coins, withdrawing fresh coins from Binance that they could easily sell elsewhere. With everyone bound to be watching Cryptopia's wallets sooner than later, I'm sure they wanted to wash them quickly.