The difference of technology between FM and AM had little or nothing to do with the big explosion, it happened because of the completely different content on the band, the counterculture was in full swing and that's where their music and culture was broadcast, the programming was unlike anything anyone had ever heard before, that is what caused the explosion.
What does IBOC have to offer that's different from current FM besides lousy range and jamming adjacent channels? Nothing besides a few HD-2 channels which sound like krap and are very hard to receive and are usually carbon copys of the main channel. It's never going to fly, even the salespeople don't know or care what it is, they know a bomb when they see it and don't sell it. I think the fuse got wet a long time ago and went out.
There are many reasons HD has been the bomb it is
No one can try it out in the few stores who actually carry any, and most people really don't care about a supposed little bit better fidelity anyway, they're expensive, and when they actually are in stores they don't work, and salespeople steer you toward satellite and/or or analog every time.
There are so many better more modern ways to get just about anything you want on the net, and people know how fast technology is moving today, WiFi will be in cars very soon. I've been listening to Classic jazz on Live 365 all night now and it costs me 6 or 7 bucks a month, already got the broadband connection.
I will never abandon Terrestrial radio but will never buy an HD radio like the vast majority of people, but will buy new methods of listening to radio and getting music. The sky will be the limit soon, HD is just a backwards diversion which if if anything is killing terrestrial radio, it's just never going to work in it's present form. If they had tried FMextra or something like that which I'm not real familiar with it may have worked as there wouldn't be so much opposition to it or maybe had tried DAB on another band where it didn't drive people nuts it may have had a chance. HD is ruining the radio that most of us love without giving us anything in return, unless of course you live in NYC and happen to be one of the 14 people who actually have an IBOC radio. I know plenty of people who own Satellite but have never met anyone who owns an iBOC receiver or has even heard of them outside of salespeople and the ones I've talked to want no part of it.
Many people including me are waiting for the next big thing and it certainly isn't IBOC. Something that works well moves fast once it gets going with little or no controversy or help. When FM started blooming in the late 60's and early 70's there was no opposition to it, it just happened all by itself like a snowball rolling down a hill. HD has been being pushed up a giant mountain by a huge loader and it still hasn't got off the ground around the mountain yet.