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.