IEO's are much safer than ICO's, especially if done in a good exchange. Exchanges have an image to watch out for, they can't make the mistake of approving bad or scam projects.
That's true, people invest not primarily because on how good the project is but on how good the exchange that will sell the coin in IEO.
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.