Well, fundmentally, it is a cost issue. Every vendor needs to be able to send disk control/selection information and not just suck back audio channels back into the head unit. Since it is going to be cheaper to bundle those 3 or 4 audio connections into a larger harness carrying the control signals vs. having two seperate sets of connectors (audio and control) including holes in the metal, mount hardware, and PCB space. Every penny saved adds up to better margins for the manufacturer.

The other element is that due to the competitive nature of branding, there is no real incentive to create compatible systems or share interface technology. If you have Brand X head unit you're "lead to" the path of buying Brand X's changer system too.... More accessories to sell = more accessories sold = more money for Brand X.

I believe there is probably a pretty consistent set of data being exchanged, but there is amply enough reasons to make it all proprietary per vendor.

What a world, what a world....
Tim