Going on a tangent, one advantage of the analog TV was that often one could watch a channel even in the presence of interference or a weak signal, whereas with the digital this is no longer possible.
Channel surfing is dead.
I woudn't trust DAB on a emergency.
That's unlikely to be the reason. A 64 kbps AAC stream definitely has more potential than FM. If the DAB station sounds worse than FM station, it's probably because they use the same sound processing for both which is still optimized for FM.