Car manufacturers are probably waiting until radio manufacturers make radios which are capable of receiving both DAB and DAB+ (at a reasonable price).
The UK is one of only two countries in Europe that have adopted the 'DAB' standard for digital radio. The rest of Europe waited until the better (and non-compatible) 'DAB+' had been fully developed. Fitting DAB radios into British cars would mean that they couldn't be used abroad. What car manufacturers want, from the radio manufacturers, are radios that can switch automatically between DAB and DAB+ (while still being able to receive FM and AM signals).
See my post here for more on this subject:
http://www.theanswerb...8.html#answer-4124051
Chris