If I am not mistaken, by the end of 2018 the ICO project has been abandoned by many parties for various reasons that have emerged and turned to the IEO.
ICO = self fundraising launching, IEO = fundraising in exchange platform.
If a new starter company want to raise funds, they need to invite investors.
To invite investors, they need to prove they are a real and not scam project. Trusted exchange can prove that with some method, that's why IEO more preferable.