Square peg, round hole problem!
The older systems use a different internal I/O interface (LVDS) whereas the retina systems use either eDP or iDP which are variations of the display port standard.
This then gets into the limits of what the different standards can push. Just like your system can push an external display the systems graphics can’t push the higher scan rates of newer video standards. As an example the it can’t push a 4K display.