So, by "most" you mean TWO stations in Minneapolis.
Come to Detroit. Ride around in my truck where I can A/B Pandora with local digital FMs. The quality difference is noticeable. Then let me A/B SiriusXM where the quality difference is night and day (except on SXM classical channels which I'm told get more bandwidth). It's not being done right everywhere, but to sample one or two stations and assume every hybrid digital station therefore "sounds terrible" is irresponsible. Some broadcasters are always going to be better than others. Why not test your theory in a town where it's done correctly? Heck, try Tulsa. I tuned all over the dial in my rental last time I was there and, of the HD broadcasters, just one was a little distorted and another (AM HD) sounded like RealAudio on a dial-up internet connection. Ugh. The rest were top notch.
Why hasn't it taken off? Typical chicken-and-egg. Plus, I think - just like AM Stereo - HD came a little late to the party. There were already other avenues for alternative programming. Plus, programmers were afraid to actively promote subchannel programming for fear of stealing listeners from the main. ESPN put prime programming on ESPN2 until cable operators HAD to add it to keep up with the demand. Radio would not do that. So, few radios were sold because there was no programming and nobody put on primo programming because there weren't any radios in the market.
Sidebar: We've tried doing exclusive content. We once did a surprise meet-and-greet with country group Rascal Flatts that was only publicized on our subchannel. Had about 30 people show up which wasn't bad for 2009. We've also tracked entire albums on day of release, run live concert captures, done album giveaways and the like. We've TRIED to offer out-of-market sports (Ohio State, Central Michigan and some others) but the schools wanted the main channel or nothing. So.. it's nothing.
I have to admit, CBS is miles ahead of the curve on this if there is a curve. In our market, alone, we have a number of niche subchannels that all turn a decent profit, though - admittedly - more from online listening than HD, though we've had HD streams show up in the ratings.
The automotive cycle is a slow one. Time from inception to production is often YEARS. We finally have a significant number of automobile models with either standard or optional HD Radios. So, the concentration, now, is not going to be on subchannels so much as making the main sound better than ever. That generally starts in large markets with large operators and trickles down. It will eventually get to the two stations you're having trouble with (Or perhaps you need to contact those stations and complain). Still, there's no way to measure how many people are listening to the main via HD vs analog. So, it could be taking off in cars and we just can't tell, yet. (I should double-check, but I'm pretty certain the PPM encoding for our analog and HD1 is the same. Even, if not, it wouldn't be split out in the ratings reports I'm privy to).
Though, when you look at percentages, streaming radio isn't mainstream, either. Not by a longshot. I have yet to meet one person who streams music in his or her car. Therefore, using your math, nobody streams in their cars.
You might also try a different radio. I purchased an Inisgnia tuner to add to my rack.. and every station in town splattered and sounded distorted. Since I work in the industry and I know what the sound is like at the source, I knew that HAD to be the receiver. Benched it at work where we have tuners of every make and model. An identical Insignia fed into our testing gear using TOSLINK sounded perfect. My unit was clipping. No idea why. Sent it back.