Computer storage has changed significantly over the years in size and types of data management. As of 2014 the primary means of transferring data from one PC to another is the USB flash drive.
The flash drive comes in one type which is formatted for the USB port. Within that there are many different levels of storage as well as different shapes, sizes and brands. USB is the type of port that the device connects into which would be the primary reason why a USB flash drive would not work in your car.
You can access data on a flash drive only if you have the appropriate port. Some car stereos have available USB inputs other do not. If you do not have a stereo with the appropriate input the USB device you are using will not work. In addition, the data type you are trying to access needs to be correct. As an example you can not use a stereo to access a Microsoft Word document. If your audio file is in an unsupported format, it won't play on your car stereo.
Stereos with USB inputs
Many of the stereos that use USB inputs are intended for portable music devices such as the iPod Touch. There is a connector that usually comes with these devices in order to sync them and that can be connected to the car stereo and used for playback. Since a standard flash drive is intended for storage only and does not have a hard drive to read the information it will not work in your car unless your car supports browsing of auxiliary devices.