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It's nothing to do with technology. Or at least, not in the way you might be thinking.
The Mega-Drive was actually compatible with the Master-System.
You just needed an adaptor because of the different cartridge slot.
This was possible because the hardware was fundamentally very similar. In some cases, they had some of the same chips. The Master-System CPU and Sound chip were both in the Mega-Drive, as co-processors, making the 16-bit console fundamentally compatible with the 8-bit one.
http://en.wikipedia.org/wiki/Mega_Drive#Master_System_compatibility.
Most consoles are just incompatible on a hardware level as the technology has moved too far forward to maintain it.
The reason the Wii is compatible with Gamecube games is because on a hardware level, the Wii is just a slightly beefed up Gamecube. Just like the Mega-Drive was a beefed up Master System.
The PS2 is backwards compatible with the Playstation because it actually has the Playstation CPU inside it, the MIPS R3000A.
Likewise the early NTSC PS3's were backwards compatible with the PS2 because they included the the PS2's CPU and GPU chips.
By the time the PS3 reached PAL regions 5 months after the NTSC release, Sony had already stripped out the PS2's CPU to reduce costs, but left the GPU, so you had a hybrid of emulated and hardware compatibility.
More PS2 games would run on these PS3's compared to Xbox games on Xbox 360, with widely variable levels of quality.
4 months later, NTSC consoles were also stripped of the Emotion Engine CPU.
When MGS4 arrived in PAL regions in late 2007, the PS2's GPU was also deleted and from that point on, all future revisions of PS3 consoles had zero compatability with PS2 games.
PSOne games are entirely emulated on the PS3, but that's really rather easy to do with the hardware.
Emulation requires a huge amount of grunt compared to being compatible on a hardware level.
It's why if you try using the Gamecube/Wii emulator, Dolphin, you'll need a really high end PC to run Super Mario Galaxy at 1080p with FSAA on and a playable framerate (and it has serious sound issues with that game).