Digital is more narrow and compact and allows more efficient use of the spectrum in general. More transmissions, different uses, sharing, freeing space up for other allocations, etc. All of that is resource/demand driven. More people is more demand.
While DAB transmission is more efficient than FM, the receivers are more complex and draw more power than FM receivers. I'm not sure the tradeoff is right in this case.
I get that argument for tv transmission, but the FM bandwidth is tiny by comparison, so just doesn't seem to justify the switching pain (particularly regarding cars)