The another subtle but big problem is supply issue. WiFi/Bluetooth combo chip is mainly desgined for cellphones and tablets, usually have product lifecycle 3-5 years. In order to be competitive (smaller footprint, lower power consumption, higher throughtput / latest standard), no chip manufacture want to stick with old generation process.
Of course car industry is very different. They want to secure parts supply at least for 10 years. For chip suppliers, that means they have to keep old generation process fab running for long time, or must have large dead stock - either way raising the cost, reducing their profit.
Unless they made sure about huge growth oppotunity in WiFi - for car market, or they suffers market satulation in consumer electoronics WiFi (PC, Phone, Tablet, etc), it is high risk (and perhaps not very profitable) challenge for chip suppliers to jump in to car industry, I think.
I want to point one thing; Bluetooth +HS mode (which is introduced at BT3.0) is actually WiFi. It is officially called PAL/AMP (Protocol Adaptation Layer / Alternative MAC and PHY), which is essentially "hijacking" WiFi chip by Bluetooth driver stack. So adapting BT+HS technology is essentially "switching Bluetooth to WiFi" as the result.
Bluetooth itself has its own "high speed" mode called EDR (Enhanced Data Rate), which was introduced at BT2.0, but EDR is only 3Mbps max.
It's kind of shocking to think that carmakers would not embrace Wi-Fi Bluetooth combo chips. As you say, Junko, nearly all smartphones use these chips. Why would automakers not fully embrace the joys of wireless connectivity?