Probably because DAB has been designed to be spectrally efficient, allowing one to cram as many radio channel as possible into a slice of spectrum. It's a property of error control codes, as used in DAB: that the closer one gets to optimal performance, the sharper the cutoff between working and not working. As range increases: works, works, works, BAM, gone...
If it's optimal, the quality of a digital link will be pretty well perfect up until that cutoff (but no link is truly optimal). DAB's design placed emphasis on more channels rather than longer range, so the point at which DAB cuts out will likely be before the point that FM has degraded to unintelligibly. In theory there can be a region where FM has degraded but DAB is still holding up, but in the name of more channels this region is probably small.
If it's optimal, the quality of a digital link will be pretty well perfect up until that cutoff (but no link is truly optimal). DAB's design placed emphasis on more channels rather than longer range, so the point at which DAB cuts out will likely be before the point that FM has degraded to unintelligibly. In theory there can be a region where FM has degraded but DAB is still holding up, but in the name of more channels this region is probably small.