We don't even need many reasons as to why IEO is better than ICO, for the fact that IEO is controlled by popular exchanges is a plus for investors because this exchanges have built reputation for themselves and will not want to spoil it for some few amount of dollars, so they will make sure to checkmate every project before approving them for IEO, and for the fact that it's done on exchanges, this just means that it is also secured and to some extent free from scammers.
It is not a plus to many people, sometimes leaving the IEO in the hands of these top exchanges could be dangerous, do you think any exchange will go that far to create IEO platform just because they have the interest people at heart? They do so to in other to make more money, using peoples weakness.
They have seen the week point of many potential investors now and fear of getting scammed if they get involved, and when thy built on that, the people they said they are protecting cannot even get involved other than whales, because only a whale would have such money to gather lots of the tokens at once without given little chance to the one that are not much financially buoyant to participate.