Binance and Houbi get more investors because they are the most popular exchange in the market, and if they make mistake, investors know they will loss more than investors loss when investing in an ICO as these exchanges are billion dollar exchanges and making millions from fees alone.
Most IEO only raise millions, so exchanges would not risk their billions just for these millions.
That's the simple analysis why investors prefer to risk their money investing in IEO in bigger exchanges.