that's easy enough to explain: the IEO phenomenon emerged during a bear market and hasn't coincided with a bubble.
bitcoin investors have always been obsessed with securities investment. in 2012 and 2013, we had things like bitcoin savings & trust, havelock investments, asicminer stocks. NXT then brought forth the ICO model and ethereum popularized it. then came the 2017 ICO bubble and IEOs after that.
so IEOs might have their day yet. otherwise i wonder what the next fundraising model will be. something is gonna get hyped the same way during the next bubble. crypto investors are simply too greedy for me to assume otherwise.