The main advantages of IEO over ICO: 

1. Savings on project marketing, as there is an opportunity to directly interest existing customers of the exchange.
2. Increasing the level of trust on the part of potential investors, as the exchange assumes reputational risks, as well as conducts due diligence of the project.
3. Provided that the exchange is not decentralized, there can be no "GAS war" between the participants. At the moment, all transactions are sooner or later included in the block, but the size of the transaction fee affects the speed. Thus, in order to be the first to buy tokens, investors compete by setting high GAS limits, thereby being the first in the transaction queue.
4. For ICO, the listing is the next round after raising funding.

If we take into account these advantages, the ICO is on the verge of extinction.