they are actually used in some cars and are predicted to become more and more common with time.
The main reasons for the time it takes to adopt it are mainly cost-effectiveness and privacy-legislation issues.
The airbag sensor, for example, stores data about the conditions that led to the deployment of the airbag. This data is stored for the purpose of "covering the ass" of the auto manufacturers after they were sued for activating airbags without reason. There's even a story about someone who was convicted in court based on the data that came from the airbag sensor.
Trucks have been using Tachographs for a long time now, although the main reason for using it is convicting truckers of speeding or driving too many hours and less as an accident reconstruction tool, which is the main application of the conventional "black-box".
There are other technologies/systems out there that collect data about the vehicle and the different car systems, but they are usually there for purposes other than reconstructing acccidents. Automotive systems such as fleet management, navigation, pay-as-you-go and so on are predicted to eventually be used as "black-boxes" for reconstructing accidents, but that may take some time and will ultimately be installed in cars during manufacturing.