General Motors and Google unable to agree on the data collection requested by Google and it is the main reason the deal was off. The report said Google wanted data from their automotive partners which will help Google to improve their autonomous driving technology but some partners like GM reluctant to give the data. Some partners worried Google will use the data for advertising purpose, but Google said they are not interested selling ads for the autonomous car (maybe not yet I think).
Other than GM, BMW and Daimler also called off their negotiation with Apple over similar concern. However, Fiat Chrysler Automobile (FCA) had agreed to partner with Google. It means FCA is open enough to give information which Google need.