Lots of the whales were trading there. Especially as those exchanges also allowed crazy levels of margin trading (10x yoru stake and sometimes 50x the stake).

What put people off is that non-Chinese people weren't supposed to trade there, and western traders had to use VPNs to get past the Great Firewall. And the Chinese exchanges charged huge fees for withdrawing fiat. Which probably encouraged shorts to push the bitcoin price down, so western whales could buy cheap bitcoin and then transfer their coins out to the western exchanges, where they could sell for dollars.

We shall see of the PBOC crackdown changes anything...