1) Most FM listeners are in cars
2) hardly cars have DAB radios
3) DAB radios hardly ever work in cars
4) If they switched off FM, the car drivers would NOT by DAB replacements.
The current plan is to leave FM radio switch of "till after the next election".
I have listened to FM on my phone twice in two years. I listen in the car all the time. If FM is turned off, I would probably listen to the pirate stations on FM. I surely won't by a DAB radio. My mum has three DAB radios. It is mostly a matter of life style.
As other posters have said "Follow the money".