What do modern, current day CD Transports do that older CDP's with RCA digital out don't?

I have read a few posts saying that even a modern inexpensive CD transport can sound better with modern DACs than older, more expensive CD players equipped with digital outputs (RCA digital output).

I understand very old CD players with optical outputs only can be poorer sounding due to the optical interface. But I would be curious to learn about how a nice $2000 Sony ES or Marantz higher grade CD player from 12-15 years ago, with RCA digital out won’t work just as well feeding a DAC as say, an modern Audiolab slot loading transport for about $600.  Let's consider the older player is working without problems, like bad laser or mechanical problems.

Hopeful that someone here can explain what the new stuff has on board that works in their favor.