I had to think what cars had computers back in 1968. My 1969 VW Fastback had one, but it was an analogue computer, so I got to thinking...I'd google it. 1983-86 means it's still been around awhile. I am kind of surprised they haven't moved on to other protocols that are faster/have more bandwidth as cars move a lot more data around, but hey, it works.
If it works fast enough for the need - why bother with a different protocol? It's not only a software issue (the engineers understand CanBus talk) it's also a hardware issue - the receiver/transmitters on a mature technology are well understood and tend to get cheaper with time. There are newer protocols in the automotive world - but some of them rely on fiber communications (MOST-BUS) - which can't carry volts/current so it requires both a controlling fiber link, and also the electronic connections to things that do stuff. It's good for infotainment systems (my Porsche has it on the audio/GPS system because it has lots of bandwidth) but overkill for module control.