Re: " it is harder to spot in a car most of the time."
That's a huge part of the problem, a substantial fraction of shipped DAB are in cars or used in cars and high quality is pointless in most of them, most of the time. That's a powerful disincentive to bother upping bitrates or risking DAB+, the users with least alternatives are the ones least likely to notice an improvement.
Fixed locations are similarly affected, if your FM, DVB or IP is seriously bad there's no pressure for DAB to do much better. If it's not bad there's even less reason to use DAB at all.
DAB service providers got away with a barely tolerable service when they had no competition and still have too little competition in key market niches to improve their offer. Alternatives have now shut them out of most niches and that can only get worse. It's a dead tech.