According to their sources, the reason the talks fell through was because Apple was more interested in developing an electric car of their own, as opposed to working with BMW and possibly licensing some of their technology. It was also suggested that it was because BMW played their cards too close to their chest as they did not want to end up being a mere supplier of hardware to tech giants such as Apple.
However interestingly enough while BMW publicly states that there are no plans to jointly develop a car with Apple, a source reportedly told them that at the very least, exploratory talks between senior managers could be revived at a later date. Given that Apple has absolutely no experience when it comes to the manufacturing of cars, it would make sense for them to team up with someone, such as BMW, who has had decades of experience in the industry.
In any case take it with a grain of salt for now, but in the past BMW did state that they would be open to the idea of working with Apple if there was something to be working on. It could also very well be that Apple is just exploring the idea of creating such a car as opposed to be seriously considering turning it into a product. After all it has been years since we last heard from Apple’s rumored HD TV set, so who knows where this will lead?