CAN bus isn't an interoperable standard, it's closer to lightly structured I2C. It feels like the standards committee might be moving in that direction with the more ethernet style version they're pushing, but I need to sit down and play with it a bit.

While CAN is a standard, it's basically transport layer only. The automakers have all sorts of custom stuff on top of it that's wildly proprietary. Talk to a mechanic and they'll complain about how each car company requires them to buy their own special software suite for diagnosing issues.