Why is backwards compatibility so hard for game consoles?
Because a console is a specific piece of hardware, and games are written to exploit it precisely — so running an old game on new hardware means reproducing behaviour the new machine does not have.
Why it is harder than it sounds. Console games historically targeted fixed hardware with known timing, known memory layout and known peripheral behaviour. Developers optimised against those specifics, including relying on quirks and undocumented behaviour, because the hardware would never change. Code written for one machine may depend on characteristics nobody documented, including exact timings.
The approaches, in increasing difficulty:
Same architecture. If the new console uses a compatible processor architecture, old code can frequently run more or less directly. This is why recent console generations have had much better backwards compatibility than earlier ones — the shift to common architectures made it tractable where it previously was not.
Hardware inclusion. Early backwards-compatible consoles physically included components from the previous machine. Effective, expensive, and abandoned as a cost-saving measure — which is why compatibility disappeared partway through some console generations.
Emulation. Software reproducing the old hardware's behaviour. Accuracy is the difficulty, not speed — reproducing behaviour exactly, including timing-dependent effects, is substantially harder than running the instructions.
Per-title work. Some compatibility programmes test and patch individual titles, which is why the list is partial rather than universal.
The non-technical obstacles, which are frequently decisive:
Licensing. Music, likenesses, sports teams and brands were licensed for a period. When the licence expires, the title cannot be re-released regardless of technical feasibility — this is why particular games are absent from compatibility lists and from stores.
Rights ownership, where the publisher no longer exists or the rights are unclear.
Commercial preference for selling remasters rather than enabling old versions.
Online services that no longer exist, so a title's functionality is partial.
Why PC does not have the problem in the same way. PC games target an operating system rather than fixed hardware, and compatibility layers have been maintained for decades — though very old titles still break on assumptions about hardware and operating system behaviour.
Preservation depends on it, which is why compatibility is a cultural question as well as a convenience one.