Because changing to DAB is a rediculous waste of money in the current economic climate.
Most people listen to radio while doing something else, often in a noisey work environment so quality is not vital. Clarity of speach and battery life, however are important. Both are better on FM. Don't force those of us working in mobile situations into working in silence or listening to CD's, keep FM and even AM and Long wave. Test match special is the best radio listening possible when doing a dull job out inthe countryside.