I wonder if some of this is to do with weight reduction. To have seperate buses must add a lot of extra equipment and wiring.
This means more weight and more power required to operate the electrics and in turn means lower MPG, higher fuel consumption and a more expensive car to design and manufacture.
The net outcome is a less competitive car in a competitive market.
I've worked with software engineers working on the CAN bus, and I had the same questions as you. According to them it has to do with the amount of cabling needed as well as electrical interference. A car has a lot of cabling going all over (a couple of hundred meters in total was the number I got), and if you can put as much of it as possible on the same bus(es), you save yourself a lot of problems.
So I suppose weight could be a factor, but space seems to be the big one.
I mean "a lot" in computer terms is like 4 lbs of equipment but since the brake pads on a truck weigh in at more than that I assume its not weight related.
This means more weight and more power required to operate the electrics and in turn means lower MPG, higher fuel consumption and a more expensive car to design and manufacture.
The net outcome is a less competitive car in a competitive market.