Until now, many cryptocurrency companies conduct ICOs in order not to pay money for doing IEOs on cryptocurrency exchanges, but this is a huge mistake, because cryptocurrency exchanges have many clients who want to earn money and they are ready to invest in cryptocurrency projects, and therefore it is necessary to conduct IEOs, not ICOs. As for the popularity of IDO, everything is obvious, not a lot of cryptocurrency companies conduct IDOs and I don't think that IDO will be so popular like ICO or IEO.