In early development of SEGA Saturn, there was also a plan to make Saturn backward compatibility with Mega Drive/Genesis back then, and that's why the console have Yamaha YMF292 as sound chip and cartridge set.
The plan eventually failed and the cartridge set changed its use for memory expansion and, in some regions, specific accessories.
Imagine if it worked, SEGA Saturn would have amazing library already from its predecessor, just like PS2, Wii, GBA, or NDS.

It would... but it's understandable why it was becoming a bit of a struggle at that point.
Meanwhile, NES, SNES and N64 were not backwards compatible with one another. You could get the Super GameBoy for the SNES and there were some third-party devices to allow backwards compatibility on these consoles, but Nintendo didn't bother much with backwards compatibility as Sega did and had an easier time with it.
For example, the "Sega Neptune" was pretty much going to be a Mega Drive+32X combined console, because of how ridiculous and power-consuming the Mega Drive+Mega CD+32X looked. Why the Sega Neptune didn't include a Mega CD player, I don't know, but I figured Sega were thinking of an all-purpose Mega Drive console that only took up one socket on your power outlet. But the Sega Neptune was scrapped in favour of the Sega Saturn.
Sega
could've made the Saturn backwards compatible with the Mega CD, but, again, why bother at that point? While 207 games were made for the Mega CD, their availability varied amongst countries.
I dunno, even if the Sega Saturns specs were incompatible to play Mega CD games, they could've perhaps released a device to get it to work, or one of those game disks that once you run them, a message will say "Now open the console and replace the disk with a Mega CD disk" sort of thing.
I wondered why the Dreamcast was not backwards compatible with the Sega Saturn for the reasons they are both CD-based consoles, it wasn't until later I learned the Dreamcast was actually a GD-based console, not CD. But Sega could've probably still made it backwards compatible if they wanted to, somehow. But at that point probably was just too much of a hassle to bother.