If DAB was an overall improvement on VHF then the switch would happen automatically.
However DAB is inferior in many ways - reception is much more difficult and sound quality is often inferior. Surely these are two the the main concerns and until these are resolved DAB should continue to stagnate in it's current backwater.
If you live in one of the areas where DAB reception is good then you are OK but in many major city locations reception is difficult or even impossible.
You may have read about BBC R4 parking a BBC car outside BBC TV Centre in London and been unable to tune into R4 with DAB whilst VHF was perfect. It really is that bad.
VHF gives us proven technology with a massive installed base of perfectly good receivers. Why replace 100 million VHF sets at a cost of £5 billion when all the money will go directly to China. Surely we can use the money better on the NHS.