There are a huge number of sensors on modern cars. In addition to all the sensors involved in engine control, you run into sensors for things like climate control, ABS, airbags, security, Lane Change Assist, self-parking, trunk position and pretty much every other feature on your car. Previously things like ABS and vehicle speed would have required different sensors, but now one module is able to take the signal from the wheel speed sensors and supply them to any other module that asks for that information.
In a traditional automotive network there were all sorts of additional wires required, to properly run signal to each of the sections. This became quite complicated very quickly.